Veranstaltungen (Archiv)

zu den aktuellen Veranstaltungen

Dienstag, 30.06.2020, 15:15 Uhr (Online Event)
Oberseminar Nonlinear Dynamics
Nicolas Perkowski, FU Berlin:
Infinite regularization by noise
mehr ... Veranstaltungsort
Online Event

Abstrakt
It is a very classical yet still surprising result that noise can have a regularizing effect on differential equations. For example, adding a Brownian motion to an ODE with bounded and measurable vector field leads to a well posed equation with Lipschitz continuous flow. While the equation without noise may have none or many solutions. Classical proofs of such results are based on stochastic analysis and on the close link between the Brownian motion and the heat equation. In that derivation it is not obvious which property of the noise gives the regularization. A more recent approach by Catellier and Gubinelli leads to pathwise conditions under which regularization occurs. I will present a simplified version of their approach and use it to construct (very irregular) paths which are infinitely regularizing: after adding them to an ODE we have a unique solution and an infinitely smooth flow - even if the vector field is only a tempered distribution. This is joint work with Fabian Harang.

Weitere Informationen
Diesen Vortrag können Sie mit Zoom verfolgen unter: https://zoom.us/j/83552703765.

Veranstalter
Freie Universität Berlin
Technische Universität Berlin
WIAS Berlin
Donnerstag, 25.06.2020, 14:00 Uhr (Online Event)
Seminar Numerische Mathematik
Holger Stephan, WIAS Berlin:
An overview of Lyapunov functions for linear continuous and discrete problems including some new results
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Online Event

Abstrakt
Lyapunov functions are useful for studying the time behaviour of the solution of evolutionary equations. The classical example is the L_2 norm, which decays on the solution of a diffusion equation with Neumann boundary conditions. The corresponding operator is an operator in divergent form, which forms a symmetric positive definite Dirichlet form. This concept can be generalized to arbitrary linear evolution equations with Markov generators. These equations are exactly those which preserve the type of physical quantities describing them: Intensive quantities (Banach space of continuous functions) are averaged, extensive quantities (Banach space of Radon measures) conserve their mass and sign. It turns out that for Markov generators with real spectrum, there is always a Hilbert space whose norm is a Lyapunov function and whose corresponding Dirichlet form is symmetric and positive. This generalizes the concept of detailed balance. An analogous result applies to the normality of operators with complex spectrum. This is a joint result with Artur Stephan from RG1. Furthermore, an infinite sequence of positive Lyapunov functions -- like energy, dissipation rate and so on -- can be defined recursively. This can be understood as a generalization of the well-known Bakry-Emery theory for diffusion type equations. In the lecture both the finite dimensional discrete case and the infinite dimensional continuous case are considered. Depending on which case is better suited for understanding the nature of the problem, we switch between them in an appropriate manner.

Weitere Informationen
For zoom login details please contact Alexander Linke linke@wias-berlin.de

Veranstalter
WIAS Berlin
Mittwoch, 24.06.2020, 15:15 Uhr (Online Event)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Thomas Eiter, WIAS Berlin:
Spatially asymptotic structure of time-periodic Navier--Stokes flows
mehr ... Veranstaltungsort
Online Event

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Berliner Oberseminar “Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Mittwoch, 24.06.2020, 10:00 Uhr (Online Event)
Forschungsseminar Mathematische Statistik
Prof. Dr. Ingo Steinwart, Universität Stuttgart:
Some thoughts and questions towards a statistical understanding of DNNs
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Online Event

Abstrakt
So far, our statistical understanding of the learning mechanisms of deep neural networks (DNNs) is rather limited. Part of the reasons for this lack of understanding is the fact that in many cases the tools of classical statistical learning theory can no longer be applied. In this talk, I will present some thoughts and possible questions that may be relevant for a successful end-to-end analysis of DNNs. In particular, I will discuss the role of initialization, over-parameterization, global minima, and optimization procedures.

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Der Vortrag findet bei Zoom statt: https://zoom.us/j/159082384

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Dienstag, 23.06.2020, 15:00 Uhr (Online Event)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Sara Mazzonetto, Universität Potsdam:
Threshold Ornstein-Uhlenbeck model: Parameters estimation
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Online Event

Abstrakt
A threshold Ornstein-Uhlenbeck is a continuous-time threshold autoregressive process. It follows the Ornstein-Uhlenbeck dynamics when above or below a fixed value, yet at this threshold its coefficients can be discontinuous. We discuss (quasi)-maximum likelihood estimation of the drift parameters, both assuming continuous and discrete time observations. In the ergodic case, we derive consistency and speed of convergence in long time and high frequency of these estimators. Based on these results, we develop a test for the presence of a threshold in the dynamics. Finally, we apply these statistical tools to the short term US interest rates. The talk is based on a joint work with Paolo Pigato.

Weitere Informationen
Dieser Vortrag findet bei Zoom statt: https://zoom.us/j/492088715

Veranstalter
WIAS Berlin
Mittwoch, 17.06.2020, 10:00 Uhr (Online Event)
Forschungsseminar Mathematische Statistik
Dr. François Bachoc, Université Paul Sabatier, Toulouse:
Valid confidence intervals post-model-selection
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Online Event

Abstrakt
In this talk, I will first introduce the post-model-selection inference setting, that has recently been subject to intensive investigation. In the case of Gaussian linear regression, I will review the post-model-selection confidence intervals suggested by Berk et al (2013). These intervals are meant to cover model-dependent regression coefficients, that depend on the selected set of variables. I will present some personal contributions on an adaptation of these confidence intervals to the case where the targets of inference are linear predictors. Then, I will present an extension of these confidence intervals to non-Gaussian and non-linear settings. The suggested more general intervals will be supported by asymptotic results and numerical comparisons with other intervals recently suggested in the literature.

Weitere Informationen
Der Vortrag findet bei Zoom statt: https://zoom.us/j/159082384

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Dienstag, 16.06.2020, 15:15 Uhr (Online Event)
Oberseminar Nonlinear Dynamics
Sindre W. Haugland, TU München:
A hierarchy of symmetries: Coexistence patterns of four (and more) Stuart--Landau oscillators with nonlinear global coupling
mehr ... Veranstaltungsort
Online Event

Abstrakt
The symmetry of a system of globally coupled identical units is high: For any solution, interchanging the behavior of any two elements still guarantees a solution. A full N! permutations are possible in total, either not changing the current solution or producing a mirror-image equivalent in phase space. Together, these N! permutations form a symmetry group with many subgroups. Thus, a large number of different partially symmetric solutions might in principle exist. Here, we study N=4 Stuart-Landau oscillators with a particularly intriguing form of global coupling. Its solutions indeed exhibit symmetries corresponding to many of the subgroups of the group of all permutations. Moreover, the transitions between these solutions, together with additional bifurcations more subtly influcencing the symmetry, form an intriguing interconnected hierarchy of differentiated oscillator dynamics. We will also take a brief look at how this hierarchy develops if the ensemble size is scaled up.

Weitere Informationen
Diesen Vortrag können Sie mit Zoom verfolgen unter: https://zoom.us/j/83893529850.

Veranstalter
Freie Universität Berlin
Technische Universität Berlin
WIAS Berlin
Dienstag, 16.06.2020, 15:00 Uhr (Online Event)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Valeriy Avanesov, WIAS Berlin:
Data-driven confidence bands for distributed nonparametric regression
mehr ... Veranstaltungsort
Online Event

Abstrakt
Gaussian Process Regression and Kernel Ridge Regression are popular nonparametric regression approaches. Unfortunately, they suffer from high computational complexity rendering them inapplicable to the modern massive datasets. To that end a number of approximations have been suggested, some of them allowing for a distributed implementation. One of them is the divide and conquer approach, splitting the data into a number of partitions, obtaining the local estimates and finally averaging them. In this paper we suggest a novel computationally efficient fully data-driven algorithm, quantifying uncertainty of this method, yielding frequentist L_2- confidence bands. We rigorously demonstrate validity of the algorithm. Another contribution of the paper is a minimax-optimal high-probability bound for the averaged estimator, complementing and generalizing the known risk bounds.

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Dieser Vortrag findet bei Zoom statt: https://zoom.us/j/492088715

Veranstalter
WIAS Berlin
Mittwoch, 10.06.2020, 15:00 Uhr (Online Event)
Forschungsseminar Mathematische Statistik
Dr. Jonathan Niles-Weed, New York University:
Minimax estimation of smooth densities in Wasserstein distance
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Online Event

Abstrakt
We study nonparametric density estimation problems where error is measured in the Wasserstein distance, a metric on probability distributions popular in many areas of statistics and machine learning. We give the first minimax-optimal rates for this problem for general Wasserstein distances, and show that, unlike classical nonparametric density estimation, these rates depend on whether the densities in question are bounded below. Motivated by variational problems involving the Wasserstein distance, we also show how to construct discretely supported measures, suitable for computational purposes, which achieve the minimax rates. Our main technical tool is an inequality giving a nearly tight dual characterization of the Wasserstein distances in terms of Besov norms. Joint work with Q. Berthet.

Weitere Informationen
Der Vortrag findet bei Zoom statt: https://zoom.us/j/159082384

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Dienstag, 09.06.2020, 15:15 Uhr (Online Event)
Oberseminar Nonlinear Dynamics
Peter Koltai, FU Berlin:
Reaction coordinates (order parameters) for metastable systems
mehr ... Veranstaltungsort
Online Event

Abstrakt
We consider non-deterministic dynamical systems showing complex metastable behavior but no local separation in fast and slow coordinates. We raise the question whether and when such systems exhibit a low-dimensional parametrization supporting their effective statistical dynamics. For answering this question, we aim at finding nonlinear coordinates, called reaction coordinates (or order parameters), such that the projection of the dynamics onto these coordinates preserves the dominant time scales of the dynamics. We show that, based on a specific reducibility property, the existence of good low-dimensional reaction coordinates preserving the dominant time scales is guaranteed. Based on this theoretical framework, we develop and test a novel numerical approach for computing good reaction coordinates. The proposed algorithmic approach is fully local and thus less prone to the curse of dimension with respect to the state space of the dynamics than global methods. Hence, it is a promising method for data-based model reduction of complex dynamical systems such as?but not only?molecular dynamics.

Weitere Informationen
Diesen Vortrag können Sie mit Zoom verfolgen unter: https://zoom.us/j/84070311127.

Veranstalter
Freie Universität Berlin
WIAS Berlin
Technische Universität Berlin
Dienstag, 09.06.2020, 10:15 Uhr (Online Event)
Seminar Nichtlineare Optimierung und Inverse Probleme
Prof. Dr. Rinaldo Mario Colombo, University of Brescia, Italien:
A hyperbolic-parabolic system to model and control parasites through parasitoids
mehr ... Veranstaltungsort
Online Event

Weitere Informationen
Diesen Vortrag können Sie mit Zoom verfolgen unter: https://zoom.us/j/98493865627.

Veranstalter
WIAS Berlin
8. – 10. Juni 2020 (Online Event)
Workshop/Konferenz: 13th Annual ERC Berlin-Oxford Young Researchers Meeting on Applied Stochastic Analysis
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Online Event

Veranstalter
Technische Universität Berlin
WIAS Berlin
Donnerstag, 04.06.2020, 11:00 Uhr (Online Event)
Seminar Numerische Mathematik
Prof. Volker John, WIAS Berlin:
On the provable convergence order for the kinetic energy of FEMs for the incompressible Navier--Stokes equations
mehr ... Veranstaltungsort
Online Event

Abstrakt
The kinetic energy of a flow is proportional to the square of the L2(Ω) norm of the velocity. Given a sufficient regular velocity field u and a velocity finite element space with polynomials of degree r, then the best approximation error in L2(Ω) is of order r+1. This talk will discuss which order of convergence for the kinetic energy can be proved with robust error estimates, i.e., with estimates where the constant does not depend on inverse powers of the viscosity. To fix ideas, the first part of the talk is devoted to evolutionary scalar convection-diffusion equations. In the second part, results for robust discretizations of the time-dependent incompressible Navier--Stokes equations will be surveyed. This survey covers as well inf-sup stable pairs of finite element spaces as pressure-stabilized discretizations.

Weitere Informationen
For zoom login details please contact Alexander Linke linke@wias-berlin.de

Veranstalter
WIAS Berlin
Mittwoch, 03.06.2020, 10:00 Uhr (Online Event)
Forschungsseminar Mathematische Statistik
Prof. Ingrid van Keilegom, KU Leuven:
On a semiparametric estimation method for AFT mixture cure models
mehr ... Veranstaltungsort
Online Event

Abstrakt
When studying survival data in the presence of right censoring, it often happens that a certain proportion of the individuals under study do not experience the event of interest and are considered as cured. The mixture cure model is one of the common models that take this feature into account. It depends on a model for the conditional probability of being cured (called the incidence) and a model for the conditional survival function of the uncured individuals (called the latency). This work considers a logistic model for the incidence and a semiparametric accelerated failure time model for the latency part. The estimation of this model is obtained via the maximization of the semiparametric likelihood, in which the unknown error density is replaced by a kernel estimator based on the Kaplan-Meier estimator of the error distribution. Asymptotic theory for consistency and asymptotic normality of the parameter estimators is provided. Moreover, the proposed estimation method is compared with a method proposed by Lu (2010), which uses a kernel approach based on the EM algorithm to estimate the model parameters. Finally, the new method is applied to data coming from a cancer clinical trial.

Weitere Informationen
Der Vortrag findet bei Zoom statt: https://zoom.us/j/159082384

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Dienstag, 02.06.2020, 15:00 Uhr (Online Event)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Sebastian Riedel, WIAS Berlin:
Runge-Kutta methods for rough differential equations
mehr ... Veranstaltungsort
Online Event

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Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics

Veranstalter
WIAS Berlin
Donnerstag, 28.05.2020, 14:00 Uhr (Online Event)
Seminar Numerische Mathematik
Dr. Hang Si, WIAS Berlin:
Detri2, a 2D constrained Delaunay-Voronoi mesh generator
mehr ... Veranstaltungsort
Online Event

Abstrakt
Detri2 is a C++ program to generate triangular meshes from arbitrary polygonal domains.It uses constrained Delaunay algorithms to conform the input line segments and maintain the locally Delaunay property of the mesh toplogy. It uses Delaunay refinement algorithms to generate meshes with good angle bounds. It can generate adaptive meshes corresponding to input mesh sizing functions. In this talk, I will give a brief tutorial of the basic functionalites of Detri2. I will demonstrate the aglorithms and many of the features of Detri2 using a graphic user interface - Detri2Qt. The source code of Detri2 and Detri2Qt are available at http://www.wias-berlin.de/people/si/detri2.html

Weitere Informationen
For zoom login details please contact Alexander Linke linke@wias-berlin.de

Veranstalter
WIAS Berlin
Mittwoch, 27.05.2020, 15:15 Uhr (Online Event)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Martin Heida, WIAS Berlin:
Stochastic homogenization on randomly perforated domains
mehr ... Veranstaltungsort
Online Event

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Dienstag, 26.05.2020, 15:15 Uhr (Online Event)
Oberseminar Nonlinear Dynamics
Eckehard Schöll, TU Berlin:
Partial synchronization patterns in the brain
mehr ... Veranstaltungsort
Online Event

Weitere Informationen
Diesen Vortrag können Sie mit Zoom verfolgen unter: https://zoom.us/j/89795202774.

Veranstalter
Freie Universität Berlin
WIAS Berlin
Technische Universität Berlin
Dienstag, 26.05.2020, 15:00 Uhr (Online Event)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Alexandra Suvorikova, WIAS Berlin:
Analysis of 3D chromatin structure using Bures-Wasserstein distance
mehr ... Veranstaltungsort
Online Event

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Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics

Veranstalter
WIAS Berlin
Mittwoch, 20.05.2020, 10:00 Uhr (Online Event)
Forschungsseminar Mathematische Statistik
Chiara Amorino, Université d'Evry Paris-Saclay, France:
Invariant adaptive density estimation for ergodic SDE with jumps over anisotropic classes
mehr ... Veranstaltungsort
Online Event

Weitere Informationen
Der Vortrag findet bei Zoom statt: https://zoom.us/j/159082384

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Dienstag, 19.05.2020, 15:00 Uhr (Online Event)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Priv.- Doz. Dr. John G. M. Schoenmakers, WIAS Berlin:
Semi-tractability of optimal stopping problems via a weighted stochastic mesh algorithm
mehr ... Veranstaltungsort
Online Event

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Dieser Vortrag findet bei Zoom statt: https://zoom.us/j/492088715

Veranstalter
WIAS Berlin
Dienstag, 19.05.2020, 10:15 Uhr (Online Event)
Seminar Nichtlineare Optimierung und Inverse Probleme
Dr. Robert Lasarzik, WIAS Berlin:
Dissipative solutions-or-a short crusade against weak solutions
mehr ... Veranstaltungsort
Online Event

Weitere Informationen
Diesen Vortrag können Sie mit Zoom verfolgen unter: https://zoom.us/j/98493865627.

Veranstalter
WIAS Berlin
Donnerstag, 14.05.2020, 14:00 Uhr (Online Event)
Seminar Numerische Mathematik
Derk Frerichs, WIAS Berlin:
Basic concepts of virtual element methods and a really pressure-robust virtual element method for the Stokes problem
mehr ... Veranstaltungsort
Online Event

Abstrakt
Virtual element methods (VEM) can be seen as a modern approach to extend finite element methods to polygonal and polyhedral meshes. This talk has two aims. First, basic ideas, concepts and advantages of VEM are presented using the well known Poisson problem. The second part concerns a VEM for the Stokes problem. For the Stokes problem exactly divergence-free VEM fail when it comes to small viscosity parameters or when the continuous pressure is complicated. In this talk a modification of the VEM is presented based on Raviart--Thomas approximations of the testfunctions which renders the method really pressure-robust, i.e. locking free for very small viscosities. The construction is also interesting for hybrid hyigh-order methods on polygonal or polyhedral meshes. Numerical results round up the theoretical findings.

Weitere Informationen
For zoom login details please contact Alexander Linke linke@wias-berlin.de

Veranstalter
WIAS Berlin
Mittwoch, 13.05.2020, 11:00 Uhr (Online Event)
Seminar Interacting Random Systems
Sam Baguley, Universität Mannheim:
Perpetual Integrals for stable SDEs
mehr ... Veranstaltungsort
Online Event

Abstrakt
The theory of one-dimensional stochastic differential equations driven by Brownian motion is classical and completely understood for many several decades. For stochastic differential equations with jumps the picture is different, and even the most basic questions are only partially understood. We study the question of existence and uniqueness of weak solutions to $$dZ_t=sigma(Z_t-)dint X_t$$ driven by $$alpha$$-stable Lévy processes.

Weitere Informationen
Diesen Vortrag können Sie mit Zoom verfolgen unter: https://zoom.us/j/86368292467.

Veranstalter
WIAS Berlin
Dienstag, 12.05.2020, 15:15 Uhr (Online Event)
Oberseminar Nonlinear Dynamics
Kostya Blyuss, University of Sussex, England:
Mathematical models of epidemics: Insight for control of COVID-19
mehr ... Veranstaltungsort
Online Event

Weitere Informationen
Diesen Vortrag können Sie mit Zoom verfolgen unter: https://zoom.us/j/89675974271.

Veranstalter
Freie Universität Berlin
WIAS Berlin
Dienstag, 12.05.2020, 15:00 Uhr (Online Event)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. César A. Uribe, Massachusetts Institute of Technology:
Distributed inference for cooperative learning
mehr ... Veranstaltungsort
Online Event

Abstrakt
We study the problem of cooperative inference where a group of agents interact over a network and seek to estimate a joint parameter that best explains a set of observations. Agents do not know the network topology or the observations of other agents. We explore a variational interpretation of the Bayesian posterior density, and its relation to the stochastic mirror descent algorithm, to propose a new distributed learning algorithm. We show that, under appropriate assumptions, the beliefs generated by the proposed algorithm concentrate around the true parameter exponentially fast. We provide explicit non-asymptotic bounds for the convergence rate. Moreover, we develop explicit and computationally efficient algorithms for observation models belonging to exponential families.

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Dieser Vortrag findet bei Zoom statt: https://zoom.us/j/492088715

Veranstalter
WIAS Berlin
Dienstag, 12.05.2020, 10:15 Uhr (Online Event)
Seminar Nichtlineare Optimierung und Inverse Probleme
Robert Gruhlke, WIAS Berlin:
Domain decomposition for random partial differential equations
mehr ... Veranstaltungsort
Online Event

Weitere Informationen
Diesen Vortrag können Sie mit Zoom verfolgen unter: https://zoom.us/j/98493865627.

Veranstalter
WIAS Berlin
Donnerstag, 07.05.2020, 14:00 Uhr (Online Event)
Seminar Numerische Mathematik
Dilara Abdel, WIAS Berlin:
Comparison of modified Scharfetter-Gummel schemes for generalized drift-diffusion systems
mehr ... Veranstaltungsort
Online Event

Abstrakt
The standard model for the description of semi-classical transport of free electrons and holes in a self-consistent electric field in a semiconductor device is the van Roosbroeck system of equations. For the discretization of this system Voronoi finite volume schemes based on different choices of flux approximations are considered. The classical Scharfetter--Gummel scheme yields a thermodynamically consistent numerical flux, but cannot be used for general charge carrier statistics. This talk aims to give an impression of the comparison and analysis of modified Scharfetter--Gummel schemes to simulate generalized drift-diffusion systems.

Weitere Informationen
For zoom login details please contact Alexander Linke linke@wias-berlin.de

Veranstalter
WIAS Berlin
Donnerstag, 07.05.2020, 10:00 Uhr (Online Event)
Forschungsseminar Mathematische Modelle der Photonik
Dr. Matthias Wolfrum, WIAS Berlin:
Temporal dissipative solitons in delay-differential equations
mehr ... Veranstaltungsort
Online Event

Weitere Informationen
Diesen Vortrag können Sie mit Zoom verfolgen unter: https://zoom.us/j/81751744679.

Veranstalter
WIAS Berlin
Mittwoch, 06.05.2020, 15:15 Uhr (Online Event)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Boris Zaltzman, Ben Gurion University of the Negev, Israel:
Equilibrium electro-convective instability in electrodeposition with Butler--Volmer kinetics
mehr ... Veranstaltungsort
Online Event

Abstrakt
Isaak Rubinstein and Boris Zaltzman. In this presentation we report that equilibrium electro-convective instability is possible in concentration polarization in the course of cathodic electrodeposition. The cathode is modelled as an ideally perm-selective interface with infinite lateral conductivity. The cation transfer across the cathode/solution interface is assumed to obey the Butler--Volmer kinetics with parameters typical of copper deposition. It is shown that deviation from the local reaction equilibrium due to the final deposition reaction rate renders possible the equilibrium electro-convective instability with a critical wavelength on the scale of diffusion layer width. This scaling may be recognized as a characteristic signature of equilibrium instability as opposed to the non-equilibrium one, related to the extended space charge. This latter instability, owning to its shortwave character, is characterized by origination of critical small vortices with a wavelength considerably shorter that the width of the diffusion layer. Interaction of these small scale vortices yields their fusion through which they evolve until reach a size comparable to the width of the diffusion layer.

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Mittwoch, 06.05.2020, 10:00 Uhr (Online Event)
Forschungsseminar Mathematische Statistik
Dr. Julia Schaumburg, VU Amsterdam, Netherlands:
Dynamic clustering of multivariate panel data
mehr ... Veranstaltungsort
Online Event

Weitere Informationen
Der Vortrag findet bei Zoom statt: https://zoom.us/j/159082384

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Dienstag, 05.05.2020, 15:15 Uhr (Online Event)
Oberseminar Nonlinear Dynamics
Prof. Carlos Rocha, Instituto Superior Tecnico, Lisboa, Portugal:
Meanders, zero numbers and the cell structure of Sturm global attractors
mehr ... Veranstaltungsort
Online Event

Abstrakt
We overview recent results on the geometric and combinatorial characterization of global attractors of semiflows generated by scalar semilinear partial parabolic differential equations under Neumann boundary conditions. This is joint work with B. Fiedler.

Weitere Informationen
Diesen Vortrag können Sie mit Zoom verfolgen unter: https://zoom.us/j/83594452457.

Veranstalter
GAMM (AG UQ)
WIAS Berlin
Dienstag, 05.05.2020, 15:00 Uhr (Online Event)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Oleg Butkovsky, WIAS Berlin:
Stochastic sewing with controls and skew fractional Brownian motion
mehr ... Veranstaltungsort
Online Event

Abstrakt
I will explain how sewing with controls developed by Peter Friz & Huilin Zhang can be extended to the stochastic setup and show how this can be used to establish existence of skew fractional brownian motion and related SDEs with iregular drift.

Weitere Informationen
Dieser Vortrag findet bei Zoom statt: https://zoom.us/j/492088715

Veranstalter
WIAS Berlin
Dienstag, 05.05.2020, 14:00 Uhr (Online Event)
Seminar Numerische Mathematik
Dr. Christian Merdon, WIAS Berlin:
A novel gradient-robust, well-balanced discretisation for the compressible Stokes problem
mehr ... Veranstaltungsort
Online Event

Abstrakt
This talk suggests a novel well-balanced discretisation of the stationary compressible isothermal Stokes problem based on the concept of gradient-robustness. Gradient-robustness means that arbitrary gradient fields in the momentum balance are well-balanced by the discrete pressure gradient - if there is enough mass in the system to compensate the force. The scheme is provably convergent and asymptotic-preserving in the sense that it degenerates for low Mach numbers to a recent inf-sup stable and pressure-robust modified Bernardi--Raugel finite element method for the incompressible Stokes equations. All properties are demonstrated in numerical examples.

Weitere Informationen
For zoom login details please contact Alexander Linke linke@wias-berlin.de

Veranstalter
WIAS Berlin
Dienstag, 05.05.2020, 10:15 Uhr (Online Event)
Seminar Nichtlineare Optimierung und Inverse Probleme
Dr. Andreas Rathsfeld, WIAS Berlin:
Scatterometry and rough surfaces
mehr ... Veranstaltungsort
Online Event

Weitere Informationen
Diesen Vortrag können Sie mit Zoom verfolgen unter: https://zoom.us/j/98493865627.

Veranstalter
WIAS Berlin
Mittwoch, 29.04.2020, 15:15 Uhr (Online Event)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Dr. Boris Zaltzman, Ben-Gurion University of the Negev, Israel:
Orientation effects in hydrodynamic instability in concentration polarization
mehr ... Veranstaltungsort
Online Event

Abstrakt
Pramoda Kumar, Isaak Rubinstein and Boris Zaltzman. One dimensional steady state passage of the electric current from an aqueous electrolyte solution into a charge selective (perm-selective) solid, such as a metal electrode or ion exchange membrane, is hydro-dynamically unstable, [1]-[3]. For a long time this instability has been attributed to electro-convection related to the extended space charge which forms at the outer edge of the electric double layer at the limiting current, [4]-[10]. For the perfectly perm-selective solid with infinite conductivity this mechanism is the only possible one. Recently, it has been shown that for a non-perfectly perm-selective membrane, or for a perfectly perm-selective membrane with finite conductivity (commercial ion-exchange membranes are practically perfectly perm-selective), additional electro-convective instability mechanisms non-related to the extended space charge are possible, [11]-[13]. All of the afore mentioned studies focused on electro-convective instability mechanisms because it was recognized the gravitational mechanism could not yield instability for most practical situations (sub-millimeter electrolyte systems with concentration below one centimolar and without a forced stirring). In such systems, the passage of electric current results in formation of concentration variations near the perm-selective solid known as concentration polarization (CP). The expression of CP is a characteristic voltage current dependence with a current saturation at the limiting value (limiting current, corresponding to a nearly vanishing interface electrolyte concentration) followed by a current increase with voltage known as the over-limiting conductance regime. For some systems, transition to the over-limiting conductance is accompanied by the appearance of excessive electric noise. Commonly, over-limiting conductance sets on due to the mechanical stirring of the interface diffusion layer resulting from a hydrodynamic electroconvective instability of quiescent electric conduction. In relation to possible gravitational mechanisms, the CP-induced density stratification results both from direct concentration variation and its related non-uniform Joule heating.The temperature variations related to the latter usually do not exceed one degree Kelvin. As a result, for gravitationally unstable orientation with the depleted interface at the bottom, the Rayleigh number related to the thermally induced density stratification is usually smaller than that related to concentration variation, whereas the latter is lower than the Rayleigh--Benard instability threshold. For this reason, electro-convective mechanisms have been invoked to explain the instability, and no systematic study of the Joule heating effects in CP has been undertaken until recently (not long ago, the effect of density stratification related to concentration variation upon the electroconvective instability in CP has been addressed in Ref. [14]). Lately, this shortcoming has been remedied in the theoretical studies by Demekhin and his group, [15]. They discovered a very interesting possibility of a Joule heating related instability expected to occur for overall stable density stratification (depleted interface on the top). In this presentation we investigate this possibility both experimentally and theoretically. References 1. G. Yossifon and H. C. Chang, Phys. Rev. Lett.101, 254501 (2008). 2. S. M. Rubinstein, G. Manukyan, A. Staicu, I. Rubinstein, B. Zaltzman, R. G. H. Lammertink, F. Mugele, and M. Wessling, Phys. Rev. Lett. 101, 236101 (2008). 3. J.C. de Valenc, R. M.Wagterveld, R. G. H. Lammertink, and P.A.Tsai, Phys.Rev.E 92, 031003(R) (2015). 4. I. Rubinstein and L. Shtilman, J. Chem. Soc. Faraday Trans. II 75, 231 (1979). 5. A.V.Listovnichy, Elektrokhimia 25, 1651 (1989). 6. V. V. Nikonenko, V. I. Zabolotsky, and N. P. Gnusin, Sov. Electrochem. 25, 262 (1989). 7. I. Rubinstein and B. Zaltzman, Phys. Rev. E62, 2238 (2000). 8. I. Rubinstein and B. Zaltzman, Math. Mod. Meth. Appl. Sci. 11, 263 - 300 (2001). 9. B. Zaltzman and I. Rubinstein, J. Fluid Mech. 579, 173 (2007). 10. C.L.Druzgalski, M.B.Andersen,and A. Mani, Phys. Fluids 25, 110804 (2013). 11. I. Rubinstein and B. Zaltzman, Phys. Rev. Lett.114, 114502 (2015). 12. R. Abu-Rjal, I. Rubinstein, and B. Zaltzman, Phys. Rev. Fluids 1, 023601 (2016). 13. I. Rubinstein and B. Zaltzman, Phys.Rev. Fluids 2, 093702 (2017). 14. E. Karatay, M.B.Andersen, M.Wessling, and A. Mani, Phys. Rev. Lett.116, 194501 (2016). 15. E. A. Demekhin, S. Amiroudine, G. S. Ganchenko, and N. Y. Khasmatulina, Phys. Rev. E91, 063006 (2015).

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Mittwoch, 29.04.2020, 10:00 Uhr (Online Event)
Forschungsseminar Mathematische Statistik
Prof. Dr. Thorsten Dickhaus, Universität Bremen:
How many null hypotheses are false?
mehr ... Veranstaltungsort
Online Event

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Der Vortrag findet bei Zoom statt: https://zoom.us/j/159082384

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Dienstag, 28.04.2020, 15:15 Uhr (Online Event)
Oberseminar Nonlinear Dynamics
Dr. Markus Kantner, WIAS Berlin:
Beyond just ,,flattening the curve”: Optimal control of epidemics with purely non-pharmaceutical interventions
mehr ... Veranstaltungsort
Online Event

Veranstalter
Freie Universität Berlin
WIAS Berlin
Dienstag, 28.04.2020, 15:00 Uhr (Online Event)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Darina Dvinskikh, WIAS Berlin:
SA vs SAA for population Wasserstein barycenter calculation
mehr ... Veranstaltungsort
Online Event

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Dieser Vortrag findet bei Zoom statt: https://zoom.us/j/492088715

Veranstalter
WIAS Berlin
Dienstag, 28.04.2020, 10:15 Uhr (Online Event)
Seminar Nichtlineare Optimierung und Inverse Probleme
David Sommer, WIAS Berlin:
A variable transformation for the Hamilton Jacobi Bellman equation
mehr ... Veranstaltungsort
Online Event

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Diesen Vortrag können Sie mit Zoom verfolgen unter: https://zoom.us/j/98493865627.

Veranstalter
WIAS Berlin
Mittwoch, 22.04.2020, 15:15 Uhr (Online Event)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Dr. Boris Zaltzman, Ben Gurion University of the Negev, Israel:
Artifact of “Breakthrough” osmosis or what may be wrong with local Spiegler--Kedem--Katchalsky equations with constant coefficients
mehr ... Veranstaltungsort
Online Event

Abstrakt
Isaak Rubinstein and Boris Zaltzman. Osmosis - solution (solvent) flow through non-perfectly (perfectly) semipermeable membranes - is a fundamental classical phenomenon of major practical importance. One of its potentially useful technological applications is the Pressure Retarded Osmosis employed for energy harvesting from salinity variations. In this process the flow resulting from the osmotic pressure drop between fresh and saline water is used to drive a turbine. Unfortunately, at the current stage, in spite of its extreme simplicity and conceptual beauty, this process does not appear to be practically viable due to insufficient power efficiency. This assessment could be radically changed by operating pressure retarded osmosis the recently theoretically predicted “Breakthrough” operation mode. In this mode, the solute concentration at the interface between the porous support and the dense selective barrier layer of a non-perfect (“leaky”) asymmetric membrane employed in process decreases with the increase of draw concentration, and, thus, the impeding effect of internal concentration polarization is eliminated. The existence of this mode was predicted based on the analysis of the system of classical local Spiegler--Kedem--Katchalsky equations of membrane transport in the barrier layer employing three constant coefficients: solute permeability, solute reflection coefficient and hydraulic permeability. In a still more recent study the physical feasibility of “Breakthrough” mode was contested and doubt was casted upon the suitability of Spiegler--Kedem--Katchalsky equations with constant coefficients for modeling of Pressure Retarded Osmosis. We re-derive the Spiegler--Kedem--Katchalsky equations based on a simple capillary friction model of membrane transport in the dense barrier layer and identify the problem with the constant coefficients' assumption resulting in the occurrence of “Breakthrough” mode. Our derivation recovers the Spiegler--Kedem--Katchalsky equations in the dilute solution limit, albeit with hydraulic permeability dependent on the local solute concentration in the barrier layer. Taking into account this dependence, necessary for preserving the detailed force balance in the barrier layer, eliminates the existence of the “Breakthrough” mode.

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Mittwoch, 22.04.2020, 10:00 Uhr (Online Event)
Forschungsseminar Mathematische Statistik
Gabriel Peyré, Ecole Normale Supérieure Paris:
Scaling optimal transport for high dimensional learning
mehr ... Veranstaltungsort
Online Event

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Der Vortrag findet bei Zoom statt: https://zoom.us/j/159082384

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Dienstag, 21.04.2020, 15:00 Uhr (Online Event)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Nikita Zhivotovskiy, Google Research:
Fast classification rates in online and statistical learning with abstention
mehr ... Veranstaltungsort
Online Event

Weitere Informationen
Dieser Vortrag findet bei Zoom statt: http://zoom.us/j/492088715

Veranstalter
WIAS Berlin
Dienstag, 21.04.2020, 14:00 Uhr (Online Event)
Seminar Materialmodellierung/ Seminar Numerische Mathematik
Dr. Alfonso Caiazzo, WIAS Berlin:
Modeling of biological flows and tissues
mehr ... Veranstaltungsort
Online Event

Abstrakt
The talk shows results of ongoing research in RG 3 related to modeling of blood flows and biological tissues. After a brief introduction on medical imaging techniques - especially magnetic resonance elastography - the first part of the talk wil discuss multiscale models for the simulation of vascularized tissues, using immersed methods. As next, we will introduce the equations and the challenges in computational hemodynamics, recent results and recently started collaborations.

Weitere Informationen
For zoom login details please contact Dirk Peschka http://www.wias-berlin.de/ peschka/?lang=1

Veranstalter
WIAS Berlin
Dienstag, 14.04.2020, 15:00 Uhr (Online Event)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Alexandra Suvorikova, WIAS Berlin:
On some general pricniples behind the results on the bootstrap validity in Bures-Wasserstein space
mehr ... Veranstaltungsort
Online Event

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Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics

Veranstalter
WIAS Berlin
Dienstag, 14.04.2020, 11:00 Uhr (Online Event)
Seminar Interacting Random Systems
Prof.Dr. Wolfgang König, WIAS Berlin:
Condensation in the interacting Bose gas in the semi-classical mean-field setting
mehr ... Veranstaltungsort
Online Event

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online seminar Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Prof. Dr. Vladimir Spokoiny, WIAS Berlin:
Bayesian inference for nonlinear inverse problems
mehr ... Veranstaltungsort
Online Event

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Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics

Veranstalter
WIAS Berlin
Dienstag, 07.04.2020, 15:00 Uhr (Online Event)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Prof. Dr. Vladimir Spokoiny, WIAS Berlin:
Bayesian inference for nonlinear inverse problems
mehr ... Veranstaltungsort
Online Event

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Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics

Veranstalter
WIAS Berlin
Dienstag, 31.03.2020, 15:00 Uhr (Online Event)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Karsten Tabelow, WIAS Berlin:
Model-based geometry reconstruction (MGBR) from TEM
mehr ... Veranstaltungsort
Online Event

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Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics

Veranstalter
WIAS Berlin
Donnerstag, 12.03.2020, 13:00 Uhr (WIAS-406)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Olivier Huber, HU Berlin:
Topics in gas transport: Nash equilibrium and constrained exact boundary controllability
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
We present two results related to the transport of gas: the existence of a solution to a Generalized Nash Equilibrium Problem (GNEP) arising from the modeling of the gas market as an oligopoly, that is only the producers are players, and the consumers just react to the quantity of gas available. In a second part, the constrained exact boundary controllability of a semilinear hyperbolic PDE is investigated. The existence of an absolutely continuous solution and boundary control will be shown, under appropriate assumptions.

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
12. – 13. März 2020 (WIAS-ESH)
Workshop/Konferenz: International Workshop: Uncertainty and Fluctuations in Thermodynamics
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Donnerstag, 05.03.2020, 14:00 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Dr. Alexander Linke, WIAS Berlin:
Gradient-robustness: A new concept assuring accurate spatial discretizations for vector-valued PDEs
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Vector-valued PDEs like the incompressible Navier--Stokes equations (in primitive variables velocity and pressure) describe the time evolution of a vector-valued quantity like the momentum density. For vector-valued PDEs it is quite natural to derive formally on the continuous level a derived time evolution of the vorticity and the divergence. Accordingly, any space discretization for a vector-valued PDE (implicitly) delivers a discrete vorticity and discrete divergence equation.
While the celebrated inf-sup stability will be shown to assure an accurate discrete divergence equation, the talk will actually focus on the question, which structural properties allow for an implicitly defined accurate discrete vorticity equation. The key observation is that the L^2-orthogonality of divergence-free vector fields and gradient fields is the weak equivalent to the vector calculus identity $nabla times nabla psi = mathbf0$ for arbitrary smooth scalar fields $psi$.
In the context of the incompressible Navier-Stokes equations, the concept of pressure-robustness was introduced in 2016, in order to discriminate between space discretizations with accurate and inaccurate discrete vorticity equations. H(div)-conforming finite element spaces have been found as an important means to realize the L^2-orthogonality between discretely divergence-free vector fields and (arbitrary) gradient fields. Further, it is shown that spatial discretizations that are not pressure-robust may suffer from i) a degradation of the (preasymptotic) convergence rate, and ii) large, parameter-dependent constants in error estimates. Typical flows that benefit from pressure-robustness are quasi-hydrostatic flows, quasi-geostrophic flows and vortex-dominated high Reynolds number flows.
Last but not least, the talk shows how to extend the concept of pressure-robustness to more general vector-valued PDEs, leading to the concept of gradient-robustness. Gradient robustness assures that dominant and extreme gradient fields in a vector-valued PDE will not lead to inaccuracies in the discretization. The talk will show examples from linear elasticity and compressible (Navier--)Stokes flows at low Mach numbers and in stratified flows. Thus, connections to well-balanced schemes and WENO schemes will be drawn.

Veranstalter
WIAS Berlin
Donnerstag, 27.02.2020, 15:00 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Dr. Anton Stephan, Institut für Physik der Atmosphäre:
Modeling Navier--Stokes equations for high Reynolds number flows and complex geometries in aerodynamic applications
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
The flight of an aircraft through a turbulent environment presents a difficult numerical problem. Though it can be easily described by the Navier-Stokes equations, it is impossible to perform a direct numerical simulation resolving all physically relevant scales. For several decades, numerous different models have been developed which perform very well for very specific problems. However, since the aircraft flight includes complex geometries as well as large spatial dimensions these models fail to capture all important features of the problem.
The vision of establishing a simulation system for virtual flight in a realistic environment is addressed by the coupling of two separate flow solvers in a bi-directional manner. This enables for example the simulation of a flight through gusts, trailing vortices and clear air turbulence, including the effects on the aircraft and the roll-up of trailing vortices and their further development until final decay. A compressible Reynolds-Averaged Navier-Stokes (RANS) solver resolves the near-field around an aircraft model. An incompressible Large-Eddy Simulation (LES) solver is used to model the atmosphere around the aircraft with its wake footprint in the LES domain. This hybrid simulation setup is used to investigate complex physical problems occurring in different flight scenarios.

Veranstalter
WIAS Berlin
Donnerstag, 27.02.2020, 15:00 Uhr (WIAS-HVP-3.13)
Seminar Laserdynamik
Maksymilian-Gregor Siwik, TU Berlin:
Calculation of Andronov--Hopf-Bifurcations of a PDE using the continuation and bifurcation software Auto-07P
mehr ... Veranstaltungsort
Weierstraß-Institut, Hausvogteiplatz 11A, 10117 Berlin, 3. Etage, Raum: 3.13

Veranstalter
WIAS Berlin
Donnerstag, 27.02.2020, 14:00 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Dr. Philip Lederer, TU Wien, Österreich:
Divergence-free tangential finite element methods for incompressible flows on surfaces
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
In this work we consider the numerical solution of incompressible flows on twodimensional manifolds. Whereas the compatibility demands of the velocity and the pressure spaces are known from the flat case one further has to deal with the approximation of a velocity field that lies only in the tangential space of the given geometry. Abandoning H1-conformity allows us to construct finite elements which are -- due to an application of the Piola transformation -- exactly tangential. To reintroduce continuity (in a weak sense) we make use of (hybrid) discontinuous Galerkin techniques. To further improve this approach, H(div Γ)-conforming finite elements can be used to obtain exactly divergence-free velocity solutions. We present several new finite element discretizations. On a number of numerical examples we examine and compare their qualitative properties and accuracy.

Veranstalter
WIAS Berlin
Donnerstag, 27.02.2020, 11:00 Uhr (WIAS-ESH)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Dr. Axel Kröner, WIAS Berlin:
Optimal control of a semilinear heat equation subject to state and control constraints
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Dienstag, 25.02.2020, 15:00 Uhr (WIAS-405-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Prof. Dr. Håkon Hoel, RWTH Aachen:
Multilevel ensemble Kalman filtering algorithms
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Weitere Informationen
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics

Veranstalter
WIAS Berlin
Donnerstag, 20.02.2020, 14:00 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Prof. Wolfgang Dreyer, WIAS Berlin:
Classical tests of electro-chemistry
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Volume changes, heat of dilution, osmosis, saturation pressure decrease and boiling temperature increase of electrolytes are classical phenomena that can be used to test and adjust parameter of electrochemical modeling. In this lecture we apply the electrolyte model developed at WIAS to these phenomena in order to critically examine the underlying assumptions. The observations require generalizations of the notions of incompressibility and surface tension.

Veranstalter
WIAS Berlin
Dienstag, 18.02.2020, 13:30 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Ondřej Pártl, WIAS Berlin:
Mathematical modeling of non-isothermal compositional compressible fluid flow in zeolite bed and above its surface
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
I will present a mathematical model and numerical scheme for the simulation of non-isothermal compositional compressible fluid flow in a heterogeneous porous medium and in the coupled atmospheric boundary layer above the surface of this porous medium, where one of the flowing components adsorbs on the porous matrix. I will discuss the application of this model to the simulation of the hydration of a zeolite bed.
My model is based on the two-domain approach, i.e., the domain in which the flow occurs is divided into the porous medium subdomain and the free flow subdomain. In each subdomain, the flow is described by the corresponding balance equations for mass, momentum and energy. On the interface between the subdomains, coupling conditions are prescribed.
In both subdomains, the spacial discretization of the governing equations is carried out via the finite volume method. For time integration, we use a special time stepping procedure.

Veranstalter
WIAS Berlin
Donnerstag, 13.02.2020, 14:00 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Dr. Gabriel R. Barrenechea, University of Strathclyde, Scotland:
Low-order divergence-free finite element methods
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
In this talk I will review results on a divergence-free reconstruction of the lowest order pair for the Navier-Stokes equation. More precisely, from a stabilised P1xP0 scheme, a divergence-free velocity field is built as the result of a lift of the pressure jumps, and it is then plugged in the convective term of the momentum equation. This process provides a method that can be proven stable without the need to suppose the mesh refined enough. We first apply this idea to the transient Navier-Stokes equations, where estimates independent of the viscosity are derived. Then, the applicability of this idea is extended to a steady-state generalised Boussinesq system.

Veranstalter
WIAS Berlin
Mittwoch, 12.02.2020, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Dr. Fredi Tröltzsch, Technische Universität Berlin:
Optimal and feedback control of some reaction-diffusion equations
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
The optimal control of some systems of nonlinear reaction-diffusion equations is considered including the Nagumo and FitzHugh--Nagumo equations. These equations are known to develop traveling wave fronts, spiral waves or scroll rings as solutions. Well-posedness of the system, differentiability of the control-to-state mapping, and optimality conditions of first order are briefly sketched. In particular, the case of sparse optimal control is addressed. Moreover, the optimization of time-delays in local and nonlocal Pyragas type feedback control up to the control by measures is discussed. Various numerical examples illustrate the great diversity of geometrical patterns and their control.

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Mittwoch, 12.02.2020, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Alexandra Carpentier, Universität Magdeburg:
Adaptive inference and its relations to sequential decision making
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Adaptive inference - namely adaptive estimation and adaptive confidence statements - is particularly important in high of in nite dimensional models in statistics. Indeed whenever the dimension becomes high or infinite, it is important to adapt to the underlying structure of the problem. While adaptive estimation is often possible, it is often the case that adaptive and honest con dence sets do not exist. This is known as the adaptive inference paradox. And this has consequences in sequential decision making. In this talk, I will present some classical results of adaptive inference and discuss how they impact sequential decision making. (based on joint works with Andrea Locatelli, Matthias Loeffler, Olga Klopp and Richard Nickl)

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Dienstag, 11.02.2020, 15:15 Uhr (FU Berlin)
Oberseminar Nonlinear Dynamics
Peter Szmolyan, Technische Universität Wien, Austria:
Advances in GSPT -- Teaching new tricks to an old dog
mehr ... Veranstaltungsort
Freie Universität Berlin, Arnimallee 3, 14195 Berlin, Hinterhaus, Raum 130

Abstrakt
Due to the efforts of many people geometric singular perturbation theory (GSPT) has developed into a very successful branch of applied dynamical systems. GSPT has proven to be very useful in the analysis of an impressive collection of diverse problems from natural sciences, engineering and life sciences. Fenichel theory for normally hyperbolic critical manifolds combined with the blow-up method at non-hyperbolic points is often able to provide remarkably detailed insight into complicated dynamical phenomena, often even in a constructive way. Much of this work has been carried out in the framework of slow-fast systems in standard form, i.e. for systems with an a priori splitting into slow and fast variables. More recently GSPT turned out to be useful for systems for which the slow-fast structures and the resulting applicability of GSPT are somewhat hidden. Problems of this type include singularly perturbed systems in nonstandard form, problems depending singularly on more than one parameter and smooth systems limiting on non-smooth systems as a parameter tends to zero. Often several distinct scalings must be used to cover the dynamics of interest and matching of these different scaling regimes is carried out by the blow-up method.. In this talk I will survey these developments and highlight some ongoing activities.

Veranstalter
Freie Universität Berlin
WIAS Berlin
Donnerstag, 06.02.2020, 14:00 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Dr. Holger Stephan, WIAS Berlin:
A general concept of majorization and a corresponding Robin Hood method
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
The Robin Hood method is a numerical method for constructing a double-stochastic matrix that maps a given vector to another vector that is also given. This is possible if the preimage majorizes the image. The majorization of vectors is a special order sometimes called the Lorentz order. A double-stochastic matrix is a special case of a Markov (or stochastic) matrix. In the case of a general Markov matrix, it is not yet clear when such a construction is possible, what kind of condition similar to the majorization must be fulfilled by the given vectors, nor is a numerical method for determining the matrix known (regardless some heuristic iterative versions of the Robin-Hood method).
In the talk, we give a complete solution to all these problems, including a general direct Robin Hood method. It turns out that the construction of a Markov matrix from two given pairs of two vectors is possible if and only if the pairs satisfy a certain order condition. If one interprets the vectors as states of a physical system, then this order is exactly the order known as the natural time order or the second law of thermodynamics.

Veranstalter
WIAS Berlin
Mittwoch, 05.02.2020, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Riccardo Cristoferi, Heriot-Watt University:
Epitaxial crystal growth with adatoms
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Surface diffusion is one of the most important mechanisms driving crystal growth. When bulk diffusion is much faster than the surface one, the evolution of the profile of the crystal is described by the so called Einstein--Nernst relation. According to this law, the normal velocity of the profile is related to the chemical potential. This evolution equation has a variational flavor, in that it can be obtained as a gradient flow of a suitable free energy. Albeit usually neglected, adatoms (atoms freely diffusing on the surface of the crystal) seem to play a fundamental role in the description of the behaviour of a solid-vapor interfaces. For this reason, some years ago Fried and Gurtin introduced a system of evolution equations to describe such a situation, where adatoms are treated as a separate variable of the problem. Also such system of evolution equations can be seen as the gradient flow of an energy. In this talk a first step in the programme of studying the above mentioned evolution equations from a variational point of view is presented. In particular, the focus is on the static problem in the small mass regime, where the elastic energy is negligible. Ground states, effective energy, and phase filed approximation suitable for numerical purposes are discussed. This latter is embedded in a general framework that allows to treat similar problems for a large class of functionals. The talk is based on works in collaboration with Marco Caroccia (Roma Tor Vergata), and Laurent Dietrich (Lycée Fabert).

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Mittwoch, 05.02.2020, 11:30 Uhr (WIAS-406)
Seminar Interacting Random Systems
Richard Pymar, Birkbeck University of London:
Mixing times of exclusion processes on regular graphs
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
Place k black particles and n-k white particles on the vertices of an n vertex graph, with one per vertex. Suppose each edge rings at rate 1 independently, and when an edge rings particles at the end-points switch positions. Oliveira conjectured that this "k-particle exclusion process" has mixing time of order at most that of k independent particles. Together with Jonathan Hermon we prove a bound for regular graphs which is in general within a log log n factor from this conjecture when k>n^c and which, in certain cases, verifies the conjecture. As a result we obtain new mixing time bounds for the exclusion process on expanders and the hypercube.

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Seminar Interacting Random Systems

Veranstalter
WIAS Berlin
Mittwoch, 05.02.2020, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Tim Sullivan, Freie Universität Berlin:
A rigorous theory of conditional mean embeddings
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Conditional mean embeddings (CME) have proven themselves to be a powerful tool in many machine learning applications. They allow the efficient conditioning of probability distributions within the corresponding reproducing kernel Hilbert spaces (RKHSs) by providing a linear-algebraic relation for the kernel mean embeddings of the respective probability distributions. Both centered and uncentered covariance operators have been used to define CMEs in the existing literature. In this talk, we develop a mathematically rigorous theory for both variants, discuss the merits and problems of either, and significantly weaken the conditions for applicability of CMEs. In the course of this, we demonstrate a beautiful connection to Gaussian conditioning in Hilbert spaces.

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Montag, 03.02.2020, 14:00 Uhr (WIAS-ESH)
INSTITUTSKOLLOQUIUM
Prof. Dr.-Ing. Georg Duda, Charité Berlin, Julius Wolff Institute for Biomechanics and Musculoskeletal Regeneration:
From Mechanical Forces to Tissue Straining - How to Employ Biophysical Cues to Guide Regeneration
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Conceptually, our research aims at understanding endogenous cascades of tissue formation, cytokine signaling and cellular self-organization especially in bone. Mechanical straining and adaptation due to mechanical cues plays a central role in all these tissues. The aim of our work is to understand the mechano-biological cues of regeneration and adaptation and how they can be employed to enable healing even in tissues with impaired regenerative capacity such as muscle, cartilage or tendon. All approaches are motivated by clinical challenges, are based on in vivo patient measurements, employ basic research principles and aim at being translated into daily clinical routine. Examples of translation include innovative concepts for joint replacement procedures, angle stable fixation of implants or cell therapies for immune-modulation to empower tissue regeneration.

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Presentation slides

Veranstalter
WIAS Berlin
Mittwoch, 29.01.2020, 10:00 Uhr (WIAS-405-406)
Forschungsseminar Mathematische Statistik
Nadja Klein, Humboldt-Universität zu Berlin:
Bayesian regression copulas
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Dienstag, 28.01.2020, 15:15 Uhr (FU Berlin)
Oberseminar Nonlinear Dynamics
Martin Brokate, TU München:
Sensitivity results for rate independent evolutions
mehr ... Veranstaltungsort
Freie Universität Berlin, Arnimallee 3, 14195 Berlin, Hinterhaus, Raum 130

Abstrakt
Rate independent evolutions are evolutions whose solution operators commute with time transformations. They are inherently nonlinear and nonsmooth. We present some introduction, outlining various mathematical approaches as well as relations to other fields. We then address sensitivity, in particular the question whether the solution operators possess weak differentiability properties.

Veranstalter
Freie Universität Berlin
WIAS Berlin
Dienstag, 28.01.2020, 15:00 Uhr (WIAS-405-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Anastasia Ivanova, Moscow Institute of Physics and Technology (MIPT), Russische Föderation:
Optimization methods for resource allocation problem
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

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Forschungsseminar “Mathematische Statistik”

Veranstalter
WIAS Berlin
28. – 30. Januar 2020 (WIAS-ESH)
Workshop/Konferenz: Workshop on PDE Constrained Optimization under Uncertainty and Mean Field Games
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Mittwoch, 22.01.2020, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Joachim Rehberg, WIAS Berlin:
The numerical range of Dirichlet forms
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Mittwoch, 22.01.2020, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Jorge Mateu, Universitat Jaume I, Valencia, Spain:
Complex spatial and spatio-temporal point process dependencies: Linear ANOVA-type models, metrics and barycenters and predictive stochastic models of crime
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Dienstag, 21.01.2020, 15:00 Uhr (WIAS-405-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr Pavel Dvurechensky, WIAS Berlin:
On the complexity of optimal transport problems
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Veranstalter
WIAS Berlin
Freitag, 17.01.2020, 14:00 Uhr (WIAS-ESH)
INSTITUTSKOLLOQUIUM
Prof. Stefan Heinrich, Universität Kaiserslautern, Germany:
Quantum computing for numerical problems - algorithms and complexity
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
After a brief introduction into quantum computation we survey results on the speedup quantum algorithms can reach over classical (i.e. non-quantum) deterministic and randomized algorithms. We focus on basic numerical problems including integration, approximation and the solution of PDEs. We compare optimal convergence rates in the quantum setting with those in the deterministic and randomized classical setting. Upper bounds are obtained by suitable algorithms and their analysis, lower bounds by using the framework of information-based complexity theory.

Veranstalter
WIAS Berlin
17. Januar 2020 (WIAS-ESH)
Workshop/Konferenz: Workshop on the occasion of the retirement of Peter Mathé
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Mittwoch, 15.01.2020, 11:30 Uhr (WIAS-406)
Seminar Interacting Random Systems
Marcel Fenzl, Universität Zürich, Schweiz:
Asymptotic results for stabilizing geometric statistics
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
Even though global in nature, many geometric statistics like the germ-grain model or the $k$-nearest neighbour model can be realized as a sum of local contributions. This idea can be formalized by using the concept of stabilizing score functions. In this talk, we investigate geometric statistics arising from such stabilizing score functions for randomly chosen input points. More specifically, we analyse the asymptotic behaviour of such a statistic for general point processes with fast decaying correlation functions.

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Seminar Interacting Random Systems

Veranstalter
WIAS Berlin
Mittwoch, 15.01.2020, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Sven Wang, University of Cambridge, UK:
Convergence rates for penalised least squares estimators in PDE-constrained regression problems
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Dienstag, 14.01.2020, 15:15 Uhr (FU Berlin)
Oberseminar Nonlinear Dynamics
Frits Veerman, University of Heidelberg:
Toll roads and freeways: Defects in bilayer interfaces in the multi-component functionalised Cahn--Hilliard equation
mehr ... Veranstaltungsort
Freie Universität Berlin, Arnimallee 3, 14195 Berlin, Hinterhaus, Raum 130

Abstrakt
We study a multi-component extension of the functionalised Cahn-Hilliard equation, which provides a framework for the formation of patterns in fluid systems with multiple amphiphilic molecules. The assumption of a length scale dichotomy between two amphiphilic molecules allows the application of geometric techniques for the analysis of patterns in singularly perturbed reaction-diffusion systems. For a generic two-component system, we show that solutions to the four-dimensional connection problem provide the leading order approximation for solutions to the full eight-dimensional barrier problem, which can be obtained through a perturbative expansion in the layer width. Moreover, we show that a saddle-node bifurcation of bilayer solutions in the four-dimensional connection problem acts as a source of so-called defect solutions, i.e. solutions to the barrier problem that are not also solutions to the connection problem. The analysis combines geometric singular perturbation theory with centre manifold theory in an infinite-dimensional context.

Veranstalter
Freie Universität Berlin
WIAS Berlin
Dienstag, 14.01.2020, 15:00 Uhr (WIAS-ESH)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Alexander Gasnikov, Moscow Institute of Physics and Technology:
An overview of distributed optimization
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Donnerstag, 09.01.2020, 14:00 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Lia Strenge, Technische Universität Berlin:
A multilayer, multi-timescale model approach for economic and frequency control in power grids using Julia
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Power systems are subject to fundamental changes due to the increasing infeed of decentralized renewable energy sources and storage. The decentral nature of the new actors in the system requires new concepts for structuring the power grid, and achieving a wide range of control tasks ranging from seconds to days. Here we introduce a multilayer dynamical network model covering a wide range of control time scales. Crucially we combine a decentralized, self-organized low-level control and a smart grid layer of devices that can aggregate information from remote sources. The stability critical task of frequency control is performed by the former, the economic objective of demand matching dispatch by the latter. Having both aspects present in the same model allows us to study the interaction between the layers. Remarkably we find that adding communication in the form of aggregation does not improve the performance in the cases considered. Instead the self-organised state of the system already contains the information required to learn the demand structure in the entire power grid. The model introduced here is highly flexible, and can accommodate a wide range of scenarios relevant to future power grids. We expect that it will be especially useful in the context of low-energy microgrids with distributed generation. All simulations and numerical experiments for control design and analysis with sampling-based methods are performed in Julia 1.1.0. The overall model is implemented as stiff nonlinear ordinary differential equation (ODE) with periodic callbacks for the control actions. The ODE has dimension 4 N, where N is the number of edges of the graph representing the power grid (i.e., N feed-in/load connections). It is planned to use automatic differentiation to learn more about the overall nonlinear model.

Veranstalter
WIAS Berlin
Mittwoch, 08.01.2020, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Michiel Renger, WIAS Berlin:
Fast reaction limits via Γ-convergence of the flux rate functional
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
We consider a system of linear ODEs on a finite graph with multiple time scales, and revisit the classical problem of deriving effective equations for the evolution. If solutions of the ODEs minimise an action functional, one can study Γ-convergence of that functional, and find the effective equations as the minimiser of the limit functional. In this work we use a natural action functional that is related to the large deviations of a random particle system; the value at each vertex can then be interpreted as a particle density. Our central idea is to study densities as well as particle fluxes through each edge. This additional information allows a full decomposition of the graph into separate components, which simplifies the Γ-convergence considerably.

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Mittwoch, 08.01.2020, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Dr. Dominik Liebl, Universität Bonn:
Fast and fair simultaneous confidence bands for functional parameters
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Quantifying uncertainty using confidence regions is a central goal of statistical inference. Despite this, methodologies for confidence bands in Functional Data Analysis are underdeveloped compared to estimation and hypothesis testing. This work represents a major leap forward in this area by presenting a new methodology for constructing simultaneous confidence bands for functional parameter estimates. These bands possess a number of striking qualities: (1) they have a nearly closed-form expression, (2) they give nearly exact coverage, (3) they have a finite sample correction, (4) they do not require an estimate of the full covariance of the parameter estimate, and (5) they can be constructed adaptively according to a desired criteria. One option for choosing bands we find especially interesting is the concept of fair bands which allows us to do fair (or equitable) inference over subintervals and could be especially useful in longitudinal studies over long time scales. Our bands are constructed by integrating and extending tools from Random Field Theory, an area that has yet to overlap with Functional Data Analysis. Authors: Dominik Liebl (University Bonn) and Matthew Reimherr (Penn State University).

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Mittwoch, 18.12.2019, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Dr. Jürgen Sprekels, WIAS Berlin, Humboldt-Universität zu Berlin:
Optimal control of a Cahn--Hilliard--Darcy system with mass source modeling tumor growth
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

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Berliner Oberseminar “Nichtlineare partielle Differentialgleichungen”

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Mittwoch, 18.12.2019, 11:30 Uhr (WIAS-406)
Seminar Interacting Random Systems
Siragan Gailus, Hausdorff Institute for Mathematics/Boston University:
Homogenization of multiscale diffusion processes with small noise
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
Dynamical systems exhibiting multiple characteristic scales in space or time arise naturally as models in a great variety of applied fields. It is moreover common to incorporate random perturbations into these models in order to account for imperfect information or to capture random phenomena. We will discuss homogenization and fluctuations limit theorems for a model of diffusion type. Results on statistical inference will be introduced if time permits.

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Seminar Interacting Random Systems

Veranstalter
WIAS Berlin
Montag, 16.12.2019, 14:00 Uhr (WIAS-ESH)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
André Uschmajew, MPI Leipzig:
Optimization on low-rank manifolds
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Low-rank matrix and tensor models are important in many applications for representing and embedding high-dimensional data or functions. They typically lead to non-convex optimization problems on sets of matrices or tensors of given rank. In this talk, we give a basic introduction to the geometry of such sets and how it can be used to derive and study optimization algorithms. Compared to direct optimization of the factors in the model, the geometric approach is more intrinsic and can lead to improved methods. For a class of quadratic cost functions on matrices we also discuss how the geometric viewpoint is useful for studying the non-convex optimization landscape under low-rank constraints.

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Joint Research Seminar on Mathematical Optimization / Non-smooth Variational Problems and Operator Equations

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Donnerstag, 12.12.2019, 16:00 Uhr (WIAS-ESH)
Seminar Laserdynamik
Denis S. Goldobin, Perm State University, Russia:
Dynamics of oscillator populations beyond the Ott--Antonsen ansatz
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Donnerstag, 12.12.2019, 14:00 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Dr. Nancy Hitschfeld Kahler, Universdad de Chile, Santiago:
GPU computing and meshing
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
GPUs are an efficient and low cost alternative to CPU clusters for solving problems that are data-parallel or close to data parallel, but there are some restrictions in the current GPU hardware that must be taken into account in order to get efficient solutions. In this talk, fundamental concepts of GPU computing will be first introduced along with relevant techniques to make optimal use of GPU hardware, such as thread branching, coalesced memory, effective use of shared memory, thread mappings onto a triangular mesh, dynamic memory allocation, thread-safe exclusion mechanisms for neighboring triangles, and indeterminate decisions and traversals over the discrete graph structure of unstructured meshes. Then, an algorithm to transform any triangulation into a Delaunay triangulation and an implementation of a particle tracking algorithm that uses previous algorithm will be discussed. Finally, other relevant aspects to take into account such as gpu-mapping techniques and the ongoing work will be mentioned.

Veranstalter
WIAS Berlin
Mittwoch, 11.12.2019, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Laura Lauerbach, Universität Würzburg:
Fracture and stochastic homogenization in the passage from discrete to continuous systems
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
We consider a one dimensional particle system, interacting through nearest-neighbour Lennard--Jones potentials. The non-standard growth conditions and the convex-concave structure of the Lennard-Jones interactions allow for fracture. The interaction potentials are assumed to be randomly distributed. We study the variational limit in the framework of Γ-convergence of this chain of particles which leads to a homogenized energy density. Further, we rescale our model which results in a limiting energy of Griffith's type, consisting of an elastic part and a jump contributions. In a further approach, we study fracture at the level of the discrete energies. This is joint work with S. Neukamm (TU Dresden), M. Schäffner (Leipzig), A. Schlömerkemper (Würzburg).

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Mittwoch, 11.12.2019, 11:30 Uhr (WIAS-406)
Seminar Interacting Random Systems
Benjamin Lees, University of Bristol:
The phase transition for random loop models on trees
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
We show the existence of a sharp phase transition from non-existence to existence of infinite loops for a random loop model on d-regular trees, for all dimensions d ≥ 3. The loop model is built up by randomly placed 'crosses' and 'bars' whose relative intensity is controlled by a parameter u. We give a recursive scheme to obtain an expansion of the critical parameter in powers of 1/d, which in principle is explicit but whose combinatorial complexity grows very quickly. We were able to explicitly obtain the first 6 terms (the first two were previously found by Ueltschi and Bjornberg for the limit d → ∞), and observed that (as functions of u) they seem to have a very interesting structure. This is a joint work with Volker Betz and Johannes Ehlert.

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Seminar Interacting Random Systems

Veranstalter
WIAS Berlin
Dienstag, 10.12.2019, 15:00 Uhr (WIAS-405-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Valeriy Avanesov, WIAS Berlin:
How to gamble with non-stationary X-armed bandits and have no regrets
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Abstrakt
In X-armed bandit problem an agent sequentially interacts with environment which yields a reward based on the vector input the agent provides. The agent's goal is to maximise the sum of these rewards across some number of time steps. The problem and its variations have been a subject of numerous studies, suggesting sub-linear and sometimes optimal strategies. The given paper introduces a new variation of the problem. We consider an environment, which can abruptly change its behaviour an unknown number of times. To that end we propose a novel strategy and prove it attains sub-linear cumulative regret. Moreover, the obtained regret bound matches the best known bound for GP-UCB for a stationary case, and approaches the minimax lower bound in case of highly smooth relation between an action and the corresponding reward. The theoretical result is supported by experimental study.

Veranstalter
WIAS Berlin
Mittwoch, 04.12.2019, 10:00 Uhr (WIAS-405-406)
Forschungsseminar Mathematische Statistik
Prof. Nils Bertschinger, Goethe Universität Frankfurt a. M.:
Systemic Greeks: Measuring risk in financial networks
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Abstrakt
Since the latest financial crisis, the idea of systemic risk has received considerable interest. In particular, contagion effects arising from cross-holdings between interconnected financial firms have been studied extensively. Drawing inspiration from the field of complex networks, these attempts are largely unaware of models and theories for credit risk of individual firms. Here, we note that recent network valuation models extend the seminal structural risk model of Merton (1974). Furthermore, we formally compute sensitivities to various risk factors -- commonly known as Greeks -- in a network context. In the end, we present some numerical illustrations and discuss possible implications for measuring systemic risk as well as insurance pricing.

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Dienstag, 03.12.2019, 13:30 Uhr (WIAS-406)
Seminar Materialmodellierung
Prof. Michal Pavelka, Charles University, Prague, Tschechische Republik:
Symmetric Hyperbolic Thermodynamically Compatible (SHTC) equations within GENERIC
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

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Seminar Materialmodellierung

Veranstalter
WIAS Berlin
3. – 5. Dezember 2019 (WIAS-ESH)
Workshop/Konferenz: Workshop on Mathematics of Deep Learning
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Donnerstag, 28.11.2019, 16:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Modelle der Photonik
Dr. Markus Kantner, WIAS Berlin:
Modeling of semiconductor devices: From quantum mechanics to non-equilibrium thermodynamics
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Mittwoch, 27.11.2019, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Dr. Tomáš Roubíček, Charles University Prague, Tschechische Republik:
Fully convective models of some processes in the Earth
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Devised towards geophysical applications for modeling of the solid Earth, a model of poro-elastodynamics with inelastic strains and with convection/diffusion of water will be formulated fully in the Eulerian setting. There, concepts of gradient of the total strain rate as well as the additive splitting of the total strain rate are used, eliminating the displacement from the formulation. It relies on that the elastic strain is small while the inelastic and the total strains can be large. The energetics behind this model is derived and used for analysis as far as the existence of global weak energy-conserving solutions concerns. In some aspects, it improves a model of V. Lyakhovsky at al. to make it thermodynamically consistent and amenable for analysis. Coupling with the fluidic parts of the Earth is also possible while using the concept of elastic (so-called semi-compressible) fluids. Also magnetic phenomena will be discussed both in the solid and the fluidic parts, i.e. paleomagnetism and Earth dynamo, respectively. The talk reflects a collaboration with Giuseppe Tomassetti (Univ. Roma Tre).

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Berliner Oberseminar Nichtlineare partielle Differentialgleichungen (Langenbach-Seminar)

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
Mittwoch, 27.11.2019, 14:00 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Dr. Johannes Zimmer, University of Bath, GB:
Regularisation and analysis of Dean--Kawasaki-type equations
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
The Dean--Kawasaki-model consists of a nonlinear stochastic partial differential equation describing the evolution of the density function for a system of finitely many particles governed by Langevin dynamics. This equation is formally obtained, in a Schwartz distribution setting, on the hydrodynamic scale. As motivation for the study of this class of equations, we will show that the fluctuations they describe can, in the purely diffusive case, be linked to macroscopic diffusion operators of Wasserstein type. We then derive and analyse a suitably regularised Dean--Kawasaki-model for noninteracting particles obeying a second order Langevin equation, in one space dimension. We prove a high-probability result for the existence and uniqueness of mild solutions to this regularised Dean--Kawasaki-model. Extensions to the case of weakly interacting particles will also be described. This is joint work with Federico Cornalba and Tony Shardlow.

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Mittwoch, 27.11.2019, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Alain Celisse, University of Lille, France:
Kernelized change-points detection procedure
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Dienstag, 26.11.2019, 15:15 Uhr (FU Berlin)
Oberseminar Nonlinear Dynamics
Jia-Yuan Dai, National Center for Theoretical Sciences, Taiwan:
Hyperbolicity of Ginzburg--Landau vortex solutions
mehr ... Veranstaltungsort
Freie Universität Berlin, Arnimallee 3, 14195 Berlin, Hinterhaus, Raum 130

Abstrakt
We prove that each equilibrium of the Ginzburg-Landau equation restricted on the invariant subspace of vortex solutions is hyperbolic, that is, its associated linearization possesses nonzero eigenvalues. This result completely describes the global attractor of vortex solutions, and also yields the Ginzburg-Landau spiral waves of nodal type. This result is a joint work with Dr. Lappicy.

Veranstalter
Freie Universität Berlin
WIAS Berlin
Dienstag, 26.11.2019, 15:00 Uhr (WIAS-405-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Franz Besold, WIAS Berlin:
Manifold clustering
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Veranstalter
WIAS Berlin
Donnerstag, 21.11.2019, 11:00 Uhr (WIAS-ESH)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Jo Andrea Brüggemann, WIAS:
On the existence of solutions and solution methods for elliptic obstacle-type quasi-variational inequalities with volume constraints
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
In this talk, an elliptic obstacle-type quasi-variational inequality (QVI) with volume constraints is studied. This type of QVI is motivated by the reformulation of a compliant obstacle problem, where two elastic membranes are subject to external forces while enclosing a constant volume. The existence of solutions to this QVI is established building on fixed-point arguments and partly on the concept of Mosco-convergence. Since Mosco-convergence of the considered feasible sets usually requires complete continuity or compactness properties of the obstacle map, a two-fold approach is explored towards generalising the available existence results for the considered QVI. Based on the analytical findings, the solution of the QVI is approached by solving a sequence of variational inequalities (VIs). Each of these VIs is tackled in function space via a path-following semismooth Newton method. An a posteriori error estimator is derived towards enhancement of the algorithm's numerical performance by using adaptive finite element methods.

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Mittwoch, 20.11.2019, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Dr. Antonius Frederik Maria ter Elst, The University of Auckland, Neuseeland:
The Dirichlet-to-Neumann operator on C 1+κ -domains
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
We present some recent results on kernel bounds for the semigroup generated by the Dirichlet-to-Neumann operator when the underlying operator has Hölder continuous coefficients and the domain has a C 1+κ-boundary. The proof depends on Gaussian bounds for derivatives of the semigroup kernel of an elliptic operator with Dirichlet boundary conditions. As a consequence the Dirichlet-to-Neumann semigroup is holomorphic on the right half-plane on L¹. Moreover, it is also strongly continuous on the space of continuous functions on the boundary and holomorphic on the right half-plane.

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Mittwoch, 20.11.2019, 11:30 Uhr (WIAS-HVP-3.13)
Seminar Interacting Random Systems
D. R. Michiel Renger:
Dynamical Phase Transitions on Finite Graphs
mehr ... Veranstaltungsort
Weierstraß-Institut, Hausvogteiplatz 11A, 10117 Berlin, 3. Etage, Raum: 3.13

Abstrakt
We consider systems where particles jump through edges of a finite graph with nonlinear intensities, and study time-averaged particle fluxes as both the number of particles and the end time go to infinity. The corresponding large-deviation rate functional involve a minimisation over trajectories with a given time-averaged flux. The minimisation can often be restricted to constant paths, resulting in a very simple expression for the rate functional. However, for specific models and specific regimes of the average flux, such expression overshoot the large-deviation rate functional, which typically happens if small oscillations in the trajectories are more profitable. In that case we say that a dynamical phase transition occurs. The literature on dynamical phase transition is almost exclusively restricted to models where the graph becomes continuous in the limit, yielding quadratic rate functionals. In our work we focus on graphs that remain discrete in the limit, which leads to entropic rate functionals. We present conditions that rule out dynamical phase transition, and for zero-range processes on a discrete ring, we give sufficient conditions under which a dynamical phase transition can be constructed. (Joint work with Davide Gabrielli, l'Aquila)

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Seminar Interacting Random Systems

Veranstalter
WIAS Berlin
Mittwoch, 20.11.2019, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Nikita Zhivotowskii, Google Zürich, Switzerland:
Robust covariance estimation for vectors with bounded kurtosis
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Let X be a centered random vector and assume that we want to estimate its covariance matrix. In this talk I will discuss the following result: if the random X satisfies the bounded kurtosis assumption, there is a covariance matrix estimator that given a sequence of n independent random vectors distributed according to X exhibits the optimal performance one would expect had X been a gaussian vector. The procedure also improves the current state-of-the-art regarding high probability bounds in the sub-gaussian case (sharp results were only known in expectation or with constant probability). In both scenarios the new bound does not depend explicitly on the dimension, but rather on the effective rank of the covariance matrix of X. The talk is based on the joint work with S. Mendelson "Robust covariance estimation under L4-L2 moment equivalence", to appear in AoS 2019.

Veranstalter
WIAS Berlin
Universität Potsdam
Humboldt-Universität zu Berlin
Dienstag, 19.11.2019, 15:15 Uhr (FU Berlin)
Oberseminar Nonlinear Dynamics
Ralf Toenjes, University of Potsdam:
The constructive role of noise in the dynamics on network hubs for network synchronization
mehr ... Veranstaltungsort
Freie Universität Berlin, Arnimallee 3, 14195 Berlin, Hinterhaus, Raum 130

Abstrakt
We describe and analyze a coherence resonance phenomenon for synchronization in bipartite networks of well connected hubs and followers when the hubs are subjected to noise. Using the Ott-Antonsen ansatz for globally coupled phase oscillators the dynamics of the mean fields is described by a low-dimensional system of Langevin equations. Averaging over the fast stochastic dynamics of the hubs yields ordinary differential equations which predict the coherence resonance reasonably well.

Veranstalter
Freie Universität Berlin
WIAS Berlin
Dienstag, 19.11.2019, 15:00 Uhr (WIAS-405-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Yangwen Sun, Humboldt-Universität zu Berlin:
Online change-point detection for high-dimensional data using graphs
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Veranstalter
WIAS Berlin
Mittwoch, 13.11.2019, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Michael Kniely, Institute of Science and Technology Austria (IST Austria) , Österreich:
On the large-time behavior of a class of semiconductor equations
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
This talk will be concerned with the exponential convergence to equilibrium of solutions to some semiconductor models arising from different modeling perspectives. The resulting PDE systems typically include drift-diffusion terms for electrons and holes as well as reaction terms describing the underlying electron-hole recombination mechanism. Including also the self-consistent electrostatic potential is often desired from a physical point of view. The main goal of the talk is the derivation of functional entropy-entropy dissipation inequalities in a constructive way for a set of semiconductor equations with and without potential and including different reaction processes. To this end, we will revisit nowadays classical calculations for prototype systems, and we shall also investigate more elaborate strategies necessary to deal with more complex models. Some parts of the talk are based on joint work with Klemens Fellner.

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Berliner Oberseminar Nichtlineare partielle Differentialgleichungen (Langenbach-Seminar)

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Mittwoch, 13.11.2019, 10:00 Uhr (WIAS-405-406)
Forschungsseminar Mathematische Statistik
Merle Behr, University of California, Berkeley:
Learning compositional structures
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Dienstag, 12.11.2019, 15:15 Uhr (FU Berlin)
Oberseminar Nonlinear Dynamics
Prof. Dr. Nicolas Perkovski, Freie Universität Berlin:
From hopping particles to stochastic PDEs
mehr ... Veranstaltungsort
Freie Universität Berlin, Arnimallee 3, 14195 Berlin, Hinterhaus, Raum 130

Abstrakt
I will try to give an overview of some of my research interests, focusing on certain a priori ill posed stochastic PDEs and their derivation. An important task in stochastics is to find and construct üniversal" models that describe a given phenomenon. For example, any random variable that is given by the superposition of many small independent influences is approximately Gaussian, independently of the concrete nature of the small influences, and therefore we call the Gaussian distribution universal. When trying to derive universal models for phenomena that evolve in space and time, formal calculations often suggest that we should consider nonlinear stochastic PDEs driven by space-time white noise. This is a problem because due to the irregularity of the noise the solution might be too irregular to make sense of the nonlinearities in the equation. But in recent years we found new ways of overcoming these problems, making sense of the equations, and proving their universality in some cases.

Veranstalter
Freie Universität Berlin
WIAS Berlin
Dienstag, 12.11.2019, 15:00 Uhr (WIAS-405-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Olga Krivorotko, Russian Academy of Sciences Novosibirsk:
Regularization of multi-parametric inverse problems for differential equations arising in immunology, epidemiology and economy
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Veranstalter
WIAS Berlin
Donnerstag, 07.11.2019, 14:00 Uhr (WIAS-405-406)
Seminar Numerische Mathematik
Benoît Gaudeul, Université de Lille , Frankreich:
Some numerical schemes for a reduced case of a Nernst--Planck--Poisson model
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Abstrakt
We compare several di erent numerical strategies to simulate a Nernst--Planck--Poisson model introduced in [1]. Different equivalent formulations are exploited based either on concentrations or activities [2]. As a first step, we focus on a simplified model for ionic liquid, for which four different schemes are discussed. A particular attention is given to the preservation at the discrete level of key features of the continuous problem, that are the positivity of the concentrations and the dissipation of the energy along time. The existence and convergence of a discrete solutions to the nonlinear system corresponding to each scheme is then established. Finally, numerical comparisons between the schemes are provided.

References
[1] Wolfgang Dreyer, Clemens Guhlke, and Rüdiger Müller. Overcoming the shortcomings of the Nernst--Planck model. Physical Chemistry Chemical Physics, 15, 2013.
[2] Jürgen Fuhrmann. Comparison and numerical treatment of generalised Nernst--Planck models. Computer Physics Communications, 6, 2015.

Veranstalter
WIAS Berlin
6. – 8. November 2019 (WIAS-ESH)
Workshop/Konferenz: Foundations of Modern Statistics
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
DFG Research Unit 1735
International Research Training Group 1792
Mittwoch, 06.11.2019, 11:30 Uhr (WIAS-HVP-3.13)
Seminar Interacting Random Systems
Tal Orenshtein, WIAS:
Random walks in random environment as rough paths
mehr ... Veranstaltungsort
Weierstraß-Institut, Hausvogteiplatz 11A, 10117 Berlin, 3. Etage, Raum: 3.13

Abstrakt
Random walk in random environment (RWRE) is a model to describe propagation of heat or diffusion of matter through a highly irregular medium. The latter is expressed locally in the model in terms of a random environment according to which the process evolves randomly in time. In a few fundamental classes the phenomenon of homogenization of the media takes place. One way this can be expressed is in the fact that on large scales, the RWRE looks like a Brownian motion with a deterministic covariance matrix given in terms the (law of the) environment. Rough path theory enables the construction of solutions to SDEs so that the solution map is continuous with respect to the noise. One important application guarantees that if the approximation converges to the noise in the rough path topology, the SDEs driven by the noise approximations converge, in an appropriate sense, to a well-defined SDE different than the original one, so that the correction term is explicit in terms of the noise approximation. In this talk we shall present our current program, in which one lifts RWRE in various classes to the rough path space and shows a convergence to an enhanced Brownian motion in the rough path topology. Interestingly, the limiting second level of the lifted RWRE may have a linear correction, called area anomaly, which we identify. Except for the immediate application to approximations of SDEs (and potentially of SPDEs), this adds some new information on the RWRE limiting path. Time permitted, we shall elaborate on the tools to tackle these problems. Based on joint works with Olga Lopusanschi, with Jean-Dominique Deushcel and Nicolas Perkowski and with Johaness Bäumler, Noam Berger, and Martin Slowik.

Weitere Informationen
Seminar Interacting Random Systems

Veranstalter
WIAS Berlin
Mittwoch, 06.11.2019, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Charles Manski, North Western University, USA:
Patient care under uncertainty (Hermann Otto Hirschfeld Lecture 2019)
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Dienstag, 05.11.2019, 15:15 Uhr (FU Berlin)
Oberseminar Nonlinear Dynamics
Phillipo Lappicy, Universidade de Sao Paulo, FU Berlin:
Hořava--Lifshitz gravity: Bifurcations and chaos
mehr ... Veranstaltungsort
Freie Universität Berlin, Arnimallee 3, 14195 Berlin, Hinterhaus, Raum 130

Veranstalter
Freie Universität Berlin
WIAS Berlin
Dienstag, 05.11.2019, 15:00 Uhr (WIAS-405-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Roman Kravchenko, WIAS und HU Berlin:
Optimal transport for imaging
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Veranstalter
WIAS Berlin
Montag, 04.11.2019, 14:00 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Dr. Christopher Rackauckas, Massachusetts Institute of Technology/University of Maryland, USA:
Neural differential equations as a basis for scientific machine learning
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Scientific Machine Learning (SciML) is an emerging discipline which merges the mechanistic models of science and engineering with non-mechanistic machine learning models to solve problems which were previously intractable. Recent results have showcased how methods like Physics Informed Neural Networks (PINNs) can be utilized as a data-efficient learning method, embedding the structure of physical laws as a prior into a learnable structures so that small data and neural networks can sufficiently predict phenomena. Additionally, deep learning embedded within backwards stochastic differential equations has been shown to be an effective tool for solving high-dimensional partial differential equations, like the Hamilton-Jacobian-Bellman equation with 1000 dimensions. In this talk we will introduce the audience to these methods and show how these diverse methods are all instantiations of a neural differential equation, a differential equation where all or part of the equation is described by a latent neural network. Once this is realized, we will show how a computational tool, DiffEqFlux.jl, is being optimized to allow for efficient training of a wide variety of neural differential equations, explaining how the performance properties of these equation differ from more traditional uses of differential equations and some of the early results of optimizing for this domain. The audience will leave knowing how neural differential equations and DiffEqFlux.jl may be a vital part of next-generation scientific tooling.

Veranstalter
WIAS Berlin
Freitag, 01.11.2019, 14:00 Uhr (WIAS-ESH)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Ana Djurdjevac, FU Berlin, Institut für Mathematik:
Random PDEs on moving hypersurfaces
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
It is well-known that in a variety of applications, especially in the biological modeling, PDEs that appear can be better formulated on evolving curved domains. Most of these equations contain various parameters and often there is a degree of uncertainty regarding the given data. We investigate the uncertainty which comes from unknown parameters or geometry. In the first part we study the advection-diffusion equation with random coefficients that is posed on an evolving hypersurface. We consider both cases, uniformly bounded and log-normal distributions of the coefficient. We introduce the solution space and prove the well-posedness using Banach - Nechas - Babushka theorem. Furthermore, we will introduce and analyse the evolving surface finite element discretization of the equation, introduced by Dziuk and Elliott. In the uniformly bounded case, we show unique solvability of the resulting semi-discrete problem and prove optimal error bounds for the semi-discrete solution and Monte Carlo of its expectation. In the second part we study PDEs that evolve with a given random velocity. Utilizing the domain mapping method, we transfer the problem into a PDE with random coefficients on a fixed domain and analyse this equation. Our theoretical convergence rates are confirmed by numerical experiments. This work is supported by DFG through project AA1-3 of MATH+

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Donnerstag, 31.10.2019, 16:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Modelle der Photonik
Jan-Philipp Köster, Ferdinand-Braun-Institut Berlin:
Traveling wave model based simulation of tunable multi-wavelength diode laser and MOPA systems
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Donnerstag, 31.10.2019, 14:00 Uhr (WIAS-ESH)
INSTITUTSKOLLOQUIUM
Prof. Max Gunzburger, Florida State University, USA:
Four “better” ways to solve the Navier--Stokes equations: Simulation of Richardson pair dispersion, ensemble discretization methods, an auxiliary equation approach for UQ, and filtered regularizations
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
The facetious and self-serving title refers to four approaches for Navier-Stokes simulations. The first involves the analysis, numerical analysis, and an efficient implementation strategy for a recently proposed fractional Laplacian closure model that accounts for Richardson pair dispersion observed in turbulent flows. The second is the exploitation of accurate and widely applicable ensemble methods in settings in which multiple inputs need to be processed, as may be the case for uncertainty quantification, reduced-order modeling, and control and optimization. The third addresses the lack of regularity of solutions and the resultant loss of accuracy of approximations in the case of white or weakly correlated additive noise forcing. The fourth involves filtered spectral viscosity and hierarchical finite element methods for regularized Navier-Stokes equations.

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Institutskolloquium

Veranstalter
WIAS Berlin
Donnerstag, 31.10.2019, 10:15 Uhr (WIAS-406)
Seminar Nichtlineare Optimierung und Inverse Probleme
Dr. Grigor Nika, WIAS Berlin:
Optimal design of 3D-printed microarchitectured polymer materials exhibiting a negative Poisson's ratio
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Veranstalter
WIAS Berlin
Mittwoch, 30.10.2019, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Katharina Hopf, WIAS Berlin:
On the singularity formation and relaxation to equilibrium in 1D Fokker--Planck model with superlinear drift
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Mittwoch, 30.10.2019, 11:30 Uhr (WIAS-406)
Seminar Interacting Random Systems
Dr. Niccolo Torri, Université Paris Nanterre, Frankreich:
Directed polymer in a heavy-tail random environment
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
A classical problem in the field of disordered systems is to understand the behaviour of a directed polymer in interaction with a random environment. Mathematically, the directed polymer is modeled by a directed random walk and the random environment is a sequence of random variables which can interact with the random walk perturbing its behaviour, giving rise to super-diffusive transversal fluctuations and localisation phenomena. Understanding the typical trajectories of the the walk is a very challenging issue and several results have been obtained when the random environment has exponential moments. In this talk we consider the case of a heavy-tail random environment, that is, the environment?s distribution function decades polynomially with exponent $alpha >0$. We mainly focus on the case in which the random environment has no second moment ($alphain (0,2)$), finding the super-diffusive transversal fluctuations of the polymer as function of the parameter $alpha$. Joint work with Quentin Berger (Sorbonne Université).

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Seminar Interacting Random Suystems

Veranstalter
WIAS Berlin
Dienstag, 29.10.2019, 15:15 Uhr (FU Berlin)
Oberseminar Nonlinear Dynamics
Pavel Gurevich and Hannes Stuke, Freie Universität Berlin:
Reinforcement learning
mehr ... Veranstaltungsort
Freie Universität Berlin, Arnimallee 3, 14195 Berlin, Hinterhaus, Raum 130

Veranstalter
Freie Universität Berlin
WIAS Berlin
Dienstag, 29.10.2019, 15:00 Uhr (WIAS-405-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Tim Jahn, Universität Frankfurt am Main:
Beyond the Bakushinskii veto
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Veranstalter
WIAS Berlin
Mittwoch, 23.10.2019, 14:00 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Dr. Ansgar Jüngel, Technische Universität Wien, Österreich:
Cross-diffusion systems: from spin semiconductors to biological populations with stochastic forcing
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Many real-world applications consist of multiple components, leading on the macroscopic scale to cross-diffusion systems which consist of strongly coupled parabolic equations. The applications may be very diverse and range from spin-polarized transport in semiconductors and ion transport in cell membranes to population dynamics. In this talk, some existence results for global-in-time weak solutions is proved, based on entropy methods. This technique was already used by Herbert Gajewski, and we detail some of his ideas to prove the large-time behavior and uniqueness of weak solutions, followed by some extensions like boundedness-by-entropy and martingale solutions to stochastic cross-diffusion systems.

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Mittwoch, 23.10.2019, 11:00 Uhr (WIAS-HVP-3.13)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Dr. Martin Holler, Universität Graz, Österreich:
A variational model for learning convolutional image atoms from incomplete data
mehr ... Veranstaltungsort
Weierstraß-Institut, Hausvogteiplatz 11A, 10117 Berlin, 3. Etage, Raum: 3.13

Abstrakt
Using lifting and relaxation strategies, we present a convex variational model for learning a convolutional sparse representation of image data via a few basic atoms. Such a representation provides a model for repeating patterns, but is also of interest for classification or as structural prior. We ensure well-posedness results for the proposed model in a general inverse problems setting and provide numerical experiments, where an atom-based representation is computed from incomplete, noisy and blurry data.

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Joint Research Seminar on Mathematical Optimization / Non-smooth Variational Problems and Operator Equations

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Mittwoch, 23.10.2019, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Vladimir Spokoiny, WIAS Berlin, HU Berlin:
Bayesian inference for nonlinear inverse problems
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
We discuss the properties of the posterior for a wide class of statistical models including nonlinear generalised regression and deep neuronal networks, nonlinear inverse problems, nonparametric diffusion, error-in-operator and IV models. The new calming approach helps to treat all such problems in a unified manner and to obtain tight finite sample results about Gaussian approximation of the posterior with an explicit error bound in term of so called effective dimension.

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Dienstag, 22.10.2019, 15:00 Uhr (WIAS-405-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Oleg Butkovsky, WIAS Berlin:
Regularization by noise for SDEs and SPDEs with applications to numerical methods
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Veranstalter
WIAS Berlin
Mittwoch, 16.10.2019, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Joachim Rehberg, WIAS Berlin:
Explicit and uniform estimates for second order divergence operators on Lp spaces
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Mittwoch, 16.10.2019, 11:30 Uhr (WIAS-406)
Seminar Interacting Random Systems
Dr. Christian Mönch, Johannes-Gutenberg-Universität:
Universality of persistence exponents for self-similar processes with stationary increments
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
n 1999, G. Molchan showed that for a centered fractional Brownian motion X on the real line Prob[X(t) <1, 1 <t<T]=TH-1+o(1), where H is the index of self-similarity of X. Furthermore, he showed that the same tail exponent occurs for sevaral other path functionals of X and conjectured that it also governs the tail of Prob[L(t)<1, 1<t<T], where L is the local time at 0 of X, but was only able to prove a lower bound. In this talk, I present an entirely novel approach to persistence problems for self-similar processes with stationary increments based on Palm theory. The technique is not limited to Gaussian processes, allows us to resolve Molchan's conjecture for ANY H-self-similar process with stationary increments that admits a sufficiently smooth local time and provides a better error estimate even for the known lower bound of Prob[L(t)<1, 1<t<T] in the fractional Brownian case. Furhermore, I will discuss some work in progress concerning fractional variants of Brownian excursion-type processes.

Weitere Informationen
Seminar Interacting Random Systems

Veranstalter
WIAS Berlin
DFG Research Center MATHEON
Mittwoch, 16.10.2019, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Dr. Alexey Onatskiy, Cambridge University:
Cointegration the modern way
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Dienstag, 15.10.2019, 15:00 Uhr (WIAS-405-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Alexandra Suvorikova, Universität Potsdam:
Shape-based domain adaptation
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Veranstalter
WIAS Berlin
Mittwoch, 09.10.2019, 11:30 Uhr (WIAS-406)
Seminar Interacting Random Systems
Dr. Lorenzo Taggi, WIAS:
Phase transition in random lattice permutations
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
We consider the model of random lattice permutations, whose realisations are permutations of the vertices of Z^d and can be viewed as a system of oriented self-avoiding loops interacting via mutual exclusion, with parameter beta (”inverse temperature”) which rewards the number ”jumps”. This model attracts interest from different perspectives: it is related to the interacting quantum Bose gas through a representation which is due to Feynman, it can be viewed as representation of the dimer model (when the inverse temperature is infinite) and it is a variant of loop O(N) models. A central question for all these models is whether a regime of ”macroscopic loops” and uniformly positive "two point correlations" occurs when the inverse temperature is large enough and d > 2. This talk presents the results and the proof techniques of a recent paper where a positive answer to this question is provided.

Weitere Informationen
Seminar Interacting Random Systems

Veranstalter
WIAS Berlin
Dienstag, 08.10.2019, 15:00 Uhr (WIAS-405-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Alexander Gasnikov, Moscow Institute of Physics and Technology:
Introduction lecture in non convex optimization
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Abstrakt
In this talk we try to observe the modern numerical methods for non convex optimization problems. In particular, we concentrate on gradient type methods, high-order methods and randomization of sum type methods. Most of the results are taken from the papers that were published in the last 2-3 years.

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WIAS Berlin
Montag, 07.10.2019, 14:00 Uhr (WIAS-ESH)
Colloquium on the occasion of the 70th birthday of Dr. Johannes Elschner


Prof. Dr. Masahiro Yamamoto (University of Tokyo):
Inverse problems for first-order transport equations by Carleman estimates

Prof. Dr. Guang-Hui Hu (Beijing Computational Science Research Center):
Corner scattering theory and data-driven inversion schemes
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin

Donnerstag, 26.09.2019, 15:00 Uhr (WIAS-406)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Prof. Dr. Martin Brokate, Technische Universität München:
Sensitivity in rate independent evolutions
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
As a topic in science, rate independent evolutions have appeared more than 100 years ago; their study as a mathematical subject in its own began in the 1960's. We will present some basic issues and then discuss in particular the question of differential sensitivity, that is, whether the associated solution operators possess weak derivatives.

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Joint Research Seminar on Mathematical Optimization / Non-smooth Variational Problems and Operator Equations

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WIAS Berlin
18. – 20. September 2019 (WIAS-ESH)
Workshop/Konferenz: Optical Solitons and Frequency Comb Generation
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
9. – 13. September 2019 (WIAS-ESH)
Workshop/Konferenz: PDE 2019: Partial Differential Equations in Fluids and Solids
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
This workshop fuses expertise on the analysis of PDEs in solids, fluid dynamics, complex fluids, and interaction of fluids with solid structure. Its goal is to create new synergies among these fields in order to advance analytical methods for current research related to systems with bulk-interface interaction, geometrically nonlinear materials, and fluid-structure interaction. This concerns, e. g., Lagrangian and Eulerian descriptions, free boundaries and moving domains, and variational approaches via energy/entropy methods.

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WIAS Berlin
Mittwoch, 04.09.2019, 11:30 Uhr (WIAS-406)
Seminar Interacting Random Systems
Alexander Hinsen, WIAS:
Phase transition for the white knight model
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Weitere Informationen
Seminar Interacting Random Systems

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WIAS Berlin
Dienstag, 03.09.2019, 13:30 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Dr. Yori Fournier, Leibniz-Institut für Astrophysik Potsdam (AIP):
Non-local effects in the solar dynamo
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Model of the solar dynamo hardly reproduce the observations of the solar magnetic field. In the presence of strong magnetic field in a turbulent fully ionized medium, such as the solar interior, temporal and spacial re-correlations can occur. In the frame of mean-field MHD, these can be treated as spacial and temporal non-localities (memory effect). We found that the magnetic buoyancy occurring in the convective solar interior implies a non-linear temporal non-locality. This additional non-linearity leads to solutions that recover the morphological evolution of the solar magnetic field.

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WIAS Berlin
Montag, 19.08.2019, 15:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Modelle der Photonik
Emil Vosmar Denning, Technical University of Denmark:
Accurate few-mode models of highly structured photonic reservoirs
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

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WIAS Berlin
3. – 8. August 2019 (TU-Berlin)
Workshop/Konferenz: 6th International Conference on Continuous Optimization
mehr ... Veranstaltungsort
Technische Universität, Straße des 17. Juni 135, 10623 Berlin, Main building (H)

Weitere Informationen
https://iccopt2019.berlin/

Veranstalter
WIAS Berlin
Technische Universität Berlin
Freie Universität Berlin
Humboldt-Universität zu Berlin
Mathematical Optimization Society
Freitag, 02.08.2019, 15:15 Uhr (WIAS-ESH)
MATHEON Special Guest Lecture
Prof. Ya-Xiang Yuan, Chinese Academy of Sciences, China:
Efficient optimization algorithms for large scale data analysis
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
In this talk, two classes of problems in large scale data analysis and their optimization algorithms will be discussed. The first class focuses on composite convex program problems, where I introduce algorithms including a regularized semi-smooth Newton method, a stochastic semi-smooth Newton method and a parallel subspace correction method. The second class is on optimization with orthogonality constraints, particularly on parallelizable approaches for linear eigenvalue problems and nonlinear eigenvalue problems, and quasi-Newton type methods. Numerical results of applications, e.g., electronic structure calculations, $l_1$-regularized logistic regression problems, Lasso problems and Hartree-Fock total energy minimization problems, will be highlighted.

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Special Guest Lecture

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WIAS Berlin
Dienstag, 30.07.2019, 10:15 Uhr (WIAS-406)
Seminar Nichtlineare Optimierung und Inverse Probleme
Jonathan Trantow, Technische Universität Berlin:
A pseudo time-stepping approach for the evolution of elastic curves
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

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WIAS Berlin
Donnerstag, 25.07.2019, 10:00 Uhr (WIAS-406)
Seminar Materialmodellierung
Dr. Robert Style, ETH Zürich, Schweiz:
Arresting phase separation with polymer networks
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
Some of the most beautiful colours in nature are seen in birds that have developed materials with extremely monodisperse, colloidal microstructures (these yield vivid structural colours). Previously it has been suggested that these can be grown by a process of arrested phase separation. Here, we take inspiration from such natural materials to grow composites with a uniform microstructure via a process of phase separation in an elastic gel network. These composites consist of uniform liquid droplets embedded in an elastic gel. The size of the droplets can be easily tuned with a number of different parameters, and presents an interesting challenge for modelling. I will also discuss how this process has applications in colloidal synthesis and phase-separation processes in living cells.

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WIAS Berlin
Freitag, 12.07.2019, 11:15 Uhr (FU Berlin)
Oberseminar Nonlinear Dynamics
Toshiyuki Ogawa, Meiji University, Tokyo, Japan:
Drift bifurcation of traveling waves in a reaction-diffusion system with 3 competing species
mehr ... Veranstaltungsort
Freie Universität Berlin, Arnimallee 3, 14195 Berlin, Hinterhaus, Raum 130

Veranstalter
Freie Universität Berlin
WIAS Berlin
Donnerstag, 11.07.2019, 15:00 Uhr (WIAS-405-406)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Dr. Georg Stadler, New York University, USA:
Sparse optimal control of PDEs with uncertain coefficients
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Abstrakt
I will discuss sparse solutions of optimal control problems governed by elliptic PDEs with uncertain coefficients. Sparsity of controls is achieved by incorporating the L^1-norm of the mean of the pointwise squared controls in the objective. The main focus is on stochastic controls that share the same sparsity structure, i.e., controls that depend on the realization of the random parameters but have identical support. We propose an iterative norm reweighting formulation, which iterates over functions defined over the physical space only and thus avoids approximation of the random space. Combining a Newton method with low-rank operator approximations, this results in an efficient solution method that avoids approximation of the uncertain parameter random space. The qualitative structure of the optimal controls and the performance of the solution algorithm are studied numerically using control problems governed by the Laplace and Helmholtz equations. This is joint work with Chen Li (NYU).

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WIAS Berlin
Donnerstag, 11.07.2019, 14:00 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Arijit Hazra, Tata Institute of Fundamental Research, Indien:
Globally constraint-preserving FR/DG scheme for Maxwell's equations up to fifth order of accuracy
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Computational electrodynamics (CED), the numerical solution of Maxwell?s equations, plays an incredibly important role in several problems in science and engineering. High accuracy solutions are desired, and the discontinuous Galerkin (DG)-based methods is one of the most sophisticated ways of delivering high accuracy in numerical CED. Maxwell?s equations also have a pair of involution constraints and mimetic schemes that globally satisfy the constraints at a discrete level are also highly desirable. Keeping the above desirable properties in mind, in this work, we propose a globally divergence-conforming DG-like scheme for CED upto fifth orders of accuracy. We achieve global constraint-preservation of involution constraints of CED by collocating the electric displacement, magnetic induction and their higher order modes in the faces of the mesh. In our numerical scheme, one has to evolve some zone-centered modes in addition to the face-centered modes at fourth and higher orders of accuracy. The novel features of our schemes are: retention of higher order accuracy without any limiter even when permittivity and permeability vary by almost an order of magnitude, superior control of numerical diffusion and preservation of electromagnetic energy with strongly varying material properties in absence of conductivity. Please do not hesitate to contact me if you have any further queries. I would like to thank you for you understanding and cooperation.

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WIAS Berlin
Mittwoch, 10.07.2019, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Artur Stephan, WIAS Berlin - BMS:
On EDP--convergence for linear reaction-diffusion systems with different time scales
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Weitere Informationen
Berliner Oberseminar “Nichtlineare partielle Differentialgleichungen" (Langenbach-Seminar)

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WIAS Berlin
Berlin Mathematical School
Mittwoch, 10.07.2019, 13:00 Uhr (WIAS-ESH)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Dr. Georg Stadler, New York University, USA:
Estimation of extreme event probabilities by combining large deviation theory and PDE--constrained optimization, with application to tsunami waves
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Tsunami waves are caused by a sudden change of ocean depth (bathymetry) after an earthquake below the ocean floor. Since large tsunami waves are extreme events, they correspond to the tail part of a probability distribution, whose exploration would require impractically many samples of a Monte Carlo method. We propose an alternative method to estimate extreme probabilities using large deviation theory (LDT), which relates the probabilities of extreme events to the solutions of a one-parameter family of optimization problems. We model tsunami waves with the shallow water equations and thus these equations appear as PDE-constraints in this optimization problem. The optimization objective includes a term that measures how extreme the event is, and a term corresponding to the likelihood of certain bathymetry changes, which are modeled as a Gaussian random field. Preliminary numerical results with the 1D inviscid shallow water equation are presented. This is joint work with Shanyin Tong and Eric Vanden-Eijnden (both NYU).

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WIAS Berlin
Mittwoch, 10.07.2019, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Arnak Dalalyan, ENSAE ParisTech:
Robust estimation in structured linear models by convex programming
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

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Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Dienstag, 09.07.2019, 13:30 Uhr (WIAS-406)
Seminar Materialmodellierung
Prof. Carsten Gräser, Freie Universität Berlin, Institut für Mathematik:
Truncated nonsmooth Newton multigrid for nonsmooth minimization problems
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
Many problems originating from continuum mechanics and material sciencelead to large scale nonsmooth optimization problems after discretization in time and space. Examples are classical binary or multi-component phase field models for phase transition and separation, frictional contact problems, plasticity, and phase field-like approaches for brittle and ductile fracture. Since standard numerical methods like, e.g., multigrid are not directly applicable due to the nonsmoothness, generic nonsmooth optimization methods are frequently used for such problems which often comes at the price of reduced efficiency. In the talk we present the Truncated Nonsmooth Newton Multigrid (TNNMG) method which combines techniques from nonsmooth optimization with multigrid and domain decomposition ideas. Instead of a black box approach this is done in a structure aware fashion leading to iterative methods whose efficiency is comparable to state of the art methods for smooth problems while being robust with respect nonsmoothness. In the talk we will introduce the algorithm, discuss convergence, and present numerical examples for various applications illustrating the efficiency of the presented approach.

Veranstalter
WIAS Berlin
Mittwoch, 03.07.2019, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Pierre-Étienne Druet, WIAS Berlin:
Weak solution analysis for incompressible multicomponent flow models
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Weitere Informationen
Berliner Oberseminar “Nichtlineare partielle Differentialgleichungen" (Langenbach-Seminar)

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WIAS Berlin
Mittwoch, 03.07.2019, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Dr. Claudia Strauch, Universität Mannheim:
Concentration and nonparametric learning of diffusion processes
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
We start by discussing uniform concentration inequalities for continuous-time analogues of empirical processes and related stochastic integrals of ergodic diffusion processes. Our approach substantially relies on combining the device of martingale approximation and moment bounds which are obtained by the generic chaining method. As a concrete statistical application, we consider the question of estimating the drift function for a large class of ergodic diffusion processes. The unknown drift is supposed to belong to a nonparametric class of smooth functions of unknown order. We suggest a fully data-driven procedure which allows for rate-optimal drift estimation (with respect to sup-norm risk) and, at the same time, yields an asymptotically effiient estimator of the invariant density of the diffusion. In the last part of the talk, we sketch applications of our results to problems from stochastic control theory. One of the fundamental assumptions in stochastic control of continuous time processes is that the dynamics of the underlying (diffusion) process is known. This is, however, usually not fulfilled in practice. We study a toy model for harvesting and natural resource management, mathematically described as an impulse control problem. In variants of this model, we suggest ways to both learn the dynamics of the underlying process and control well at the same time. In particular, the combination of results from stochastic control and our previous analysis of the sup-norm risk allows to derive mathematical results for reinforcement learning.

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Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Dienstag, 02.07.2019, 13:30 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Dr. Ankik Kumar Giri, Indian Institute of Technology Roorkee:
Recent developments in the theory of coagulation-fragmentation models
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
The coagulation-fragmentation equations describe the kinetics of particle growth in which particles can coagulate via binary interaction to form larger particles or fragment to form smaller ones. These models arise in many fields of science and engineering: kinetic of phase transformations in binary alloys such as segregation of binary alloys, aggregation of red blood cells in biology, fluidized bed granulation processes, raindrop breakup in clouds, aerosol physics, i.e. the evolution of a system of solid or liquid particles suspended in a gas, formation of planets in astrophysics, polymer science and many more.
The coagulation is in general a nonlinear process where the fragmentation is classified into two major categories, one of them is the linear fragmentation and another one is the nonlinear fragmentation. The linear fragmentation may occur due to the external forces or spontaneously (that depends on the nature of particles). However, the nonlinear fragmentation takes place due to the collision between a pair of particles. Therefore, it is also known as collision-induced fragmentation or collisional breakage.
In general, the non-conservative approximation of coagulation and linear fragmentation equations may lead to the occurrence of gelation phenomenon i.e. the breakdown of mass conservation property of the solution. In the first half of this talk, it is shown that the non-conservative approximation of coagulation and linear fragmentation equations can also provide the existence of mass conserving solutions for large classes of unbounded coagulation and fragmentation kernels.The fragmentation kernel may have a singularity near the origin. Later on, this result is further generalized by including the singular coagulation kernels in the existence theory of mass-conserving solution to the nonlinear coagulation equation using non-conservative approximations.
In the second half, we introduce an existence result on weak solutions to the continuous coagulation equation with collision-induced fragmentation for certain classes of unbounded collision and breakup distribution kernels. The breakup kernel may have a possibility to attain a singularity at the origin. The proof is based on the weak compactness methods applied to suitably chosen conservative approximating equations. The question of uniqueness is also considered under additional growth conditions on the kernels which mainly relies on the integrability of higher moments. Moreover, It is observed that the unique solution is mass-conserving.

Veranstalter
WIAS Berlin
Donnerstag, 27.06.2019, 16:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Modelle der Photonik
Dr. Markus Kantner, WIAS Berlin:
Non-isothermal generalization of the Scharfetter--Gummel scheme for degenerate semiconductors
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

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WIAS Berlin
Mittwoch, 26.06.2019, 10:00 Uhr (WIAS-405-406)
Forschungsseminar Mathematische Statistik
Dr. Alexandra Suvorikova, Universität Potsdam:
On CLT in Bures-Wasserstein space and beyond
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Abstrakt
In the first part of the talk we present some concentration and convergence properties of Bures-Wasserstein (BW) barycenters of hermitian finite-dimensional matrices, and explain how they can be used for investigation of geometry of DNA molecules modelled as a union of ridgid bodies. In the second part we show how the framework of classical resampling techniques can be extended to the case of the BW space, and introduce some geometrical intuition behind the construction of non-asymptotic confidence sets for BW barycenters.

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Dienstag, 25.06.2019, 15:15 Uhr (WIAS-HVP-3.13)
Oberseminar Nonlinear Dynamics
Stefan Ruschel, TU Berlin:
Pulsing dynamics of a laser system with delayed feedback
mehr ... Veranstaltungsort
Weierstraß-Institut, Hausvogteiplatz 11A, 10117 Berlin, 3. Etage, Raum: 3.13

Abstrakt
We consider the Yamada model for an excitable or self-pulsating laser with saturable absorber in the presence of delayed optical self-feedback. We present some recent results regarding the stability and bifurcations of periodic light pulses that are sustained in the external cavity feedback loop, so called external cavity pulse trains.

Veranstalter
Freie Universität Berlin
WIAS Berlin
Dienstag, 25.06.2019, 15:00 Uhr (WIAS-405-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Fabian Telschow, University of California San Diego, USA:
Spatial confidence sets for raw effect size images
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

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WIAS Berlin
Dienstag, 25.06.2019, 13:30 Uhr (WIAS-406)
Seminar Materialmodellierung
Prof. Luca Heltai:
Unconventional frameworks for the simulation of coupled bulk-interface problems
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
Partial differential equations with interfaces, holes, cracks, or defects often require the numerical solution of coupled bulk-interface problems. In this talk, I will discuss and analyse some techniques that can be used to tackle this class of problems, using non-matching discretisations that combine finite element methods, regularization techniques, weighted Sobolev spaces, and reduce order models.

Veranstalter
WIAS Berlin
Montag, 24.06.2019, 15:15 Uhr (WIAS-HVP-3.13)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Jens Gravesen, Technical University of Denmark, Dänemark:
Differential geometry and dimension reduction for nano structures
mehr ... Veranstaltungsort
Weierstraß-Institut, Hausvogteiplatz 11A, 10117 Berlin, 3. Etage, Raum: 3.13

Abstrakt
Many nano structures are essentially one or two dimensional and it is in this case often possible to reduce the equations from three to one or two dimensions. That gives huge computational savings but can also provide insight into how the geometry of structures influences the physical properties. In the case of the Schrödinger equation for a single particle this influence is expressed in the form of a simple curvature potential. For coupled equations such as the Luttinger--Kohn Hamiltonian the procedure is no longer straight forward. The boundary conditions pose some problems and an extra approximation is necessary. We will give an overview of the method and point to some of the possible pitfalls when a choice of frame is needed.

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
24. – 26. Juni 2019 (WIAS-ESH)
Workshop/Konferenz: Workshop on Phase Transitions and Particle Systems
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Mittwoch, 19.06.2019, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Janusz Ginster, Technische Universität Berlin:
Plasticity as the Γ-limit of models for straight, parallel dislocations
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Crystal plasticity is the effect of a crystal undergoing a permanent change of shape in response to applied forces. At the atomic scale, dislocations - local defects of the crystalline lattice concentrated on lines - are considered to play a main role in this effect. In this talk, we will derive a macroscopic strain-gradient plasticity theory from two different semi-discrete models for straight, parallel edge dislocations. Firstly, we complement the existing literature by considering a rotationally invariant energy with mixed growth. The main advantage of this formulation is that it does not need the ad-hoc definition of a core-radius. In addition to the Γ-convergence result we present a rigidity result for fields with non-vanishing curl in this setting. Secondly, we will address the fact that most results in the literature are derived under the assumption that the present dislocations are separated on a mesoscopic scale. We will show how to overcome this assumption by an adaption of the ball construction technique which was developed in the context of the Ginzburg--Landau model for superconductivity.

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Mittwoch, 19.06.2019, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Dr. Harel Hron, Palacký University, Olomouc, Czech Republic:
Weighting of densities in Bayes spaces with application to simplicial functional principal component analysis
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Probability density functions (PDFs) can be understood as functional data carrying relative information. As such, standard methods of functional data analysis (FDA) are not appropriate for their statistical processing. They are typically designed in the L2 space (with Lebesgue reference measure), thus cannot be directly applied to densities, as the metrics of L2 does not honor their geometric properties. This has recently motivated the construction of the so-called Bayes Hilbert spaces, which result from the generalization of the Aitchison geometry for compositional data to the in nite dimensional setting. More precisely, if we focus on PDFs restricted to a bounded support (that is mostly used in practical applications), they can be represented with respect to the Lebesgue reference measure using the Bayes space of positive real functions with square-integrable logarithm. The reference measure can be easily changed through the well-known chain rule and interpreted as a weighting technique in Bayes spaces. Moreover, it impacts on the geometry of the Bayes spaces and results in so-called weighted Bayes spaces. The aim of this contribution is to show the e ects of changing the reference measure from the Lebesgue measure to a general probability measure focusing on its practical implications for the Simplicial Functional Principal Component Analysis (SFPCA). A centered log-ratio transformation is proposed to map a weighted Bayes spaces into an unweighted L2 space (i.e. with Lebesgue reference measure), thus it enables to apply standard statistical methods such as SFPCA on PDFs.

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Dienstag, 18.06.2019, 15:00 Uhr (WIAS-405-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Priv.-Doz. Dr. Peter Mathé, WIAS Berlin:
The modulus of continuity in Bayesian inverse problems
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Abstrakt
We shall discuss how the convergence rates of direct and inverse problems are related through the modulus of continuity.

Weitere Informationen
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics

Veranstalter
WIAS Berlin
Dienstag, 18.06.2019, 13:30 Uhr (WIAS-406)
Seminar Materialmodellierung
Prof. Amit Acharya, Carnegie Mellon University, USA:
Line defect dynamics and solid mechanics
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
Continuum mechanics has been a successful model for studying macroscopic deformations and the forces causing them. The usual framework allows the study of continuous deformations giving way to surfaces of discontinuity, but does not provide an adequate framework for considering the dynamics of the terminating lines of surfaces of discontinuity, were such to occur. It turns out that such terminating lines of surfaces of discontinuity serve as a model of common line defects that arise in a host of materials; dislocations and grain/phase boundary junctions in crystalline and soft matter. I will describe a framework for considering line defect dynamics within continuum mechanics. I will show how the kinematics of line defect dynamics provides a unifying theme for describing the defects mentioned above, resulting in an augmentation of the classical balance laws of continuum mechanics with a microscopic conservation law for topological charge carried by these defect lines. The theory will be illustrated with examples related to dislocation dynamics with inertia, the computation of fields of interfacial defects like the star disclination and grain boundary disconnections.

Weitere Informationen
Seminar Materialmodellierung

Veranstalter
WIAS Berlin
Mittwoch, 12.06.2019, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Francesco Della Porta, Max-Planck-Institut für Mathematik Leipzig:
The nonlocal Cahn--Hilliard--Hele--Shaw system with regular and singular potential
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
The Cahn--Hilliard--Hele--Shaw system (also called the Cahn--Hilliard--Darcy system) can be used to describe the evolution of binary immiscible fluids in a Hele--Shaw cell or in porous media, and has been recently used as a buildingblock for some tumor-growth models. In my talk, I will introduce a physically motivated nonlocal variant of the Cahn--Hilliard--Hele--Shaw system, and present some related analytical results. These include the well-posedness of the system, the existence of a global attractor, and the convergence of solutions to equilibria as time goes to infinity (in the two-dimensional case). We discuss both the case with regular and singular potential.

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Mittwoch, 12.06.2019, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Samuel Müller, University of Sydney, Australia:
c2pLasso: The categorical-continuous pliable Lasso to identify brain regions affecting motor impairment in Huntington disease
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
In many clinical studies, prediction models are essential for forecasting and monitoring the progression of a disease. Developing prediction models is a challenge when dealing with high-dimensional data since we do not know which variables are related to the response variable of interest and this relationship may depend on other continuous or categorical modifying variables as well. We formalize this problem as the varying-coefficient model selection and propose a novel variable selection method, c2pLasso, that accounts for both continuous and categorical modifying variables.
Our contributions are three-fold:
1. The c2pLasso method is shown to better screen irrelevant variables over the existing method that ignores the group structure of categorical modifying variables and to lead to a prediction model with higher accuracy and easier interpretation.
2. Our method adequately considers the pre-specified group structure among modifying variables in addition to unstructured modifying variables.
3. The c2pLasso is empirically shown to perform better than existing methods such as the Lasso and pLasso even when there is no categorical modifying variable or any pre-specified group structure among modifying variables. Using simulation studies, we show our method selects less irrelevant variables compared to existing methods while choosing relevant variables correctly. This provides us with a prediction model with higher specificity, lower false discovery rate and lower mean squared error. The proposed methodology is motivated by and illustrated using data from a Huntington disease study; the result identifies brain regions associated with motor impairment accounting for differentiated relationship by disease severity. To the best of our knowledge, our study is the first to identify the interaction effect between disease severity and the volume of brain regions in a varying-coefficient model framework.
This is joint work with Rakheon Kim and Tanya Garcia, both at Texas A&M, Department of Statistics.

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
11. Juni 2019 (WIAS-ESH)
Workshop/Konferenz: Colloquium in Honor of the 80th Birthday of Klaus Schneider
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Dienstag, 11.06.2019, 15:00 Uhr (WIAS-405-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Nikolas Tapia, WIAS Berlin:
Signatures in shape analysis
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Weitere Informationen
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics

Veranstalter
WIAS Berlin
Mittwoch, 05.06.2019, 15:00 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Patrick Tolksdorf, Université Paris-Est - Créteil Val-de-Marne, Frankreich:
On the Lp-theory for second-order elliptic operators in divergence-form with complex coefficients (Part 2)
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Given a complex, elliptic coefficient function we investigate for which values of p the corresponding second-order divergence form operator, complemented with Dirichlet, Neumann or mixed boundary conditions, generates a strongly continuous semigroup on Lp(Ω). Additional properties like analyticity of the semigroup, H-calculus and maximal regularity are also discussed. Finally we prove a perturbation result for real coefficients that gives the whole range of p's for small imaginary parts of the coefficients. Our results are based on the recent notion of p-ellipticity, reverse Hölder inequalities and Gaussian estimates for the real coefficients.

Weitere Informationen
Berliner Oberseminar “Nichtlineare partielle Differentialgleichungen" (Langenbach-Seminar)

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
Mittwoch, 05.06.2019, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Yegor Klochkov, Humboldt-Universität zu Berlin:
Influencer dynamics in opinion networks
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Dienstag, 04.06.2019, 13:30 Uhr (WIAS-406)
Seminar Materialmodellierung
Dr. Giselle Antunes Monteiro, Tschechische Akademie der Wissenschaften, Prag, Tschechische Republik:
On the convergence of viscous approximation for rate-independent processes with regulated inputs
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
The vanishing viscosity method is a popular tool for describing rate-independent evolution. It consists in the analysis of the limiting behavior of a regularized problem obtained by introducing a viscous dissipation mechanism which stabilizes the process. In this talk, we discuss some issues related to viscous approximations to rate-independent processes when different choices of the viscosity operator are considered. We show that the viscous limit exists, and the associated input-output operator is continuous in the space of regulated functions. Notably, we observe that the vanishing viscosity limit may exhibit some unexpected behavior when the input has some jump discontinuities.

Veranstalter
WIAS Berlin
Montag, 03.06.2019, 11:30 Uhr (WIAS-ESH)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Dr. Guozhi Dong, Humboldt-Universität zu Berlin:
Quantitative magnetic resonance imaging: From fingerprinting to integrated physics-based models
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
In this talk, we introduce a novel method for quantitative MRI. The proposed approach simultaneously recovers the tissue parameters as the recently developed method magnetic resonance fingerprinting (MRF). However, in comparison with MRF and many of its variants, our new model is dictionary free. Thus, its accuracy is independent of the discretization size of the dictionary. The efficiency of our new method is proved theoretically and also verified by numerical examples.

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Joint Research Seminar on Mathematical Optimization / Non-smooth Variational Problems and Operator Equations

Veranstalter
WIAS Berlin
Montag, 03.06.2019, 11:00 Uhr (WIAS-ESH)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Selma Metzner, PTB / TU Berlin:
Approximate large-scale Bayesian inference with application to magnetic resonance fingerprinting
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
A class of nonlinear, large-scale regression problems is considered where the parameters model the spatial distribution of some property. Specific assumptions about the regression function are made and a homoscedastic Gaussian sampling distribution is assumed. The class of problems includes the task of Magnetic Resonance Fingerprinting (MRF), a new approach for quantitative Magnetic Resonance Imaging that allows for the estimation of absolute values of tissue related parameters like proton density and spin relaxation times. The properties of a Bayesian inference for the considered class of regression problems is investigated when different types of priors are employed, including Gaussian Markov random field priors expressing spatial smoothness. The properties of the resulting posteriors are explored and conditions for propriety of the posterior and existence of its moments established. An approximate calculation scheme is proposed and its practicability demonstrated up to dimensions of 105. Finally, the approach is applied to MRF. In using simulated data with known ground truth, it is shown that by using the prior knowledge of smoothness in the spatial distribution of the sought parameters, the results are significantly better than those achieved through maximum likelihood estimation.

Veranstalter
WIAS Berlin
Mittwoch, 29.05.2019, 15:00 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Patrick Tolksdorf, Université Paris-Est - Créteil Val-de-Marne, Frankreich:
On the Lp-theory for second-order elliptic operators in divergence-form with complex coefficients (Part 1)
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Given a complex, elliptic coefficient function we investigate for which values of p the corresponding second-order divergence form operator, complemented with Dirichlet, Neumann or mixed boundary conditions, generates a strongly continuous semigroup on Lp(Ω). Additional properties like analyticity of the semigroup, H-calculus and maximal regularity are also discussed. Finally we prove a perturbation result for real coefficients that gives the whole range of p's for small imaginary parts of the coefficients. Our results are based on the recent notion of p-ellipticity, reverse Hölder inequalities and Gaussian estimates for the real coefficients.

Weitere Informationen
Berliner Oberseminar “Nichtlineare partielle Differentialgleichungen" (Langenbach-Seminar)

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
Mittwoch, 29.05.2019, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Denis Belomestny, Universität Duisburg-Essen:
Density deconvolution under general assuptions on measurement error distribution
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
The subject of my talk is the density deconvolution problem under general assumptions on the measurement error distribution. Typically deconvolution estimators are constructed using Fourier transform techniques, and the main assumption is that the measurement error characteristic function does not vanish on the real line. This assumption is rather strong and does not hold in many cases of interest. Here we develop a new technique to deal with this problem which allows us to recover the standard convergence rates without additional assumptions on the measurement error distribution. Joint work with A. Goldenshluger (University of Haifa).

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Dienstag, 28.05.2019, 15:15 Uhr (WIAS-ESH)
Oberseminar Nonlinear Dynamics
Sebastian Eydam, WIAS Berlin:
Mode-locked solutions and coherence echoes in systems of globally-coupled phase oscillators
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
In systems of Kuramoto type globally-coupled phase oscillators with equidistant natural frequencies, one can observe an interesting collective phenomenon below the synchronization threshold. The collective behavior is characterized by sharp pulses in the mean-field amplitude and therefore appropriately called mode locked. We discuss the emergence of this particular type of solution as well as the typical bifurcation scenarios that are found along their stability boundaries. In large ensembles, where the natural frequencies follow from a multi-modal distribution, mode-locked solutions are observed and the breakdown of the pulsation due to the broadening of the frequency width is explored. Echo phenomena have a long history and are found in various systems with one of the most prominent examples being the appearance of spin-echoes. We consider an echo-type response phenomenon that occurs after multiple external stimuli in ensembles of phase oscillators below the synchronization threshold. An intuitive explanation of the coherence echo phenomenon in the language of mode locking is given and it is shown that the nonlinear global interaction that supports mode-locked solutions can also enhance the echo response significantly.

Veranstalter
WIAS Berlin
Freie Universität Berlin
Dienstag, 28.05.2019, 15:00 Uhr (WIAS-405-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Paul Hager, TU Berlin:
The multiplicative chaos of fractional Brownian motions with vanishing Hurst parameters
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Abstrakt
Empirical estimates attest the stochastic volatility process a small path regularity, which in the context of the rough volatility models are estimates of a small Hurst parameter of the underlying fractional Brownian motion. Ever smaller estimates have motivated Neuman and Rosenbaum to investigate the limit of fractional Brownian motions with vanishing Hurst parameters. Due to the vanishing Hölder regularity, the convergence can only be meaningful in the sense of generalised functions. The authors presented a normalised sequence of fractional Brownian motions and showed the convergence towards a log-correlated field, i.e. a generalised Gaussian field with a logarithmic singularity on the diagonal of the covariance kernel. Placed in the abstract ambient of the less known fractional Gaussian fields, we find the essential property of the normalisation that makes this convergence possible and we pose a general class of normalisations, which apply to fractional Brownian fields as well. The more interesting object from the finance perspective is the volatility process, i.e the exponential of the fractional Brownian motion. The limit process is not point-wise defined, yet we can make sense of the integrated volatility process as a random measure. The theory of Gaussian multiplicative chaos deals exactly with the latter problem: Defining the exponential of singular Gaussian fields. The usual construction of these measures uses a regularisation of the field, therefore most results are not directly applicable in our situation. This problem was treated by Neuman and Rosenbaum, however their proofs were incomplete. We are able to state an elementary proof for the convergence of the volatility measures in the so call L2-phase.

Veranstalter
WIAS Berlin
Dienstag, 28.05.2019, 10:15 Uhr (WIAS-406)
Seminar Nichtlineare Optimierung und Inverse Probleme
Dr. Björn Sprungk, Georg-August-Universität Göttingen:
Noise-level robust sampling methods for Bayesian inverse problems
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Veranstalter
WIAS Berlin
Donnerstag, 23.05.2019, 16:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Modelle der Photonik
Dr. Maria Chernysheva, Leibniz Institute of Photonic Technology, Jena:
Highlights of pulse generation and dynamics in mode-locked all-fibre and SOA-based fibre lasers
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
23. – 25. Mai 2019 (WIAS-ESH)
Workshop/Konferenz: 11th Annual ERC Berlin-Oxford Young Researchers Meeting on Applied Stochastic Analysis
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Technische Universität Berlin
University of Oxford
Mittwoch, 22.05.2019, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Pierluigi Colli, Università di Pavia, Italien:
Well-posedness, regularity and asymptotic analyses for a fractional phase field system
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Mittwoch, 22.05.2019, 11:15 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Gilian Heller, Macquarie University, Sydney:
Part II: Ordinal regression models for continuous scales
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Visual Analogue Scales (VAS) are used for measuring quantities which are intangible and difficult to measure on conventional scales, such as pain, anxiety and quality of life. These are generally used for self-rating. Subjects are given a linear scale of 100 mm and asked to put a mark where they perceive themselves. The scale has verbal anchor descriptors at each extreme, such as (in the pain context) ?no pain? and ?worst pain imaginable?. The VAS reading is taken as the measurement from the left endpoint to the subject's mark, and is usually normalized to lie in the interval [0,1]. Statistical analysis of the VAS is controversial. While it is a bounded, continuous variable, several authors have argued that it is ordinal, rather than ratio in nature, and should be treated as such. The issue is that, for example, a 1-cm difference in VAS scores at the lower end of the scale does not necessarily represent the same difference in the intangible outcome as a 1-cm difference at the upper end; and a doubling of VAS score may not translate to a doubling of e.g. the pain or anxiety. This problem is overcome by treating VAS measurements as ordinal rather than ratio data. We therefore refer to scales of this type as continuous ordinal. We have developed a regression framework for continuous ordinal responses. We express the likelihood in terms of a function connecting the scale with an underlying continuous latent variable and approximate this function non-parametrically. Then a general semi-parametric regression framework for continuous scales is developed. The model is shown to be a conditional transformation model, and is generalizable to a much wider range of uses than the context in which it was developed. We illustrate our method on a quality of life data set.

Veranstalter
Universität Potsdam
Humboldt-Universität zu Berlin
WIAS Berlin
Mittwoch, 22.05.2019, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Gilian Heller, Macquarie University, Sydney:
Part I: Parameter orthogonality and the GAMLSS family of distributions
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Parameter orthogonality is a desirable property of statistical distributions having more than one parameter. When parameters are orthogonal then their maximum likelihood estimates are asymptotically independent. Within the exponential family, the mean and dispersion parameter are orthogonal; however in general this is not the case. This work is motivated by a trial in Parkinson's disease patients in which one of the outcomes is the number of falls. Inspection of the data reveals that the Poisson-inverse Gaussian (PiG) distribution is appropriate, and that the experimental treatment reduces not only the mean, but also the variability, substantially. Conventional analysis assumes a treatment effect on the mean, either adjusted or unadjusted for covariates, and a constant dispersion parameter. We find that we reach quite different conclusions on the treatment effect on the mean, depending on whether or not a model is specified for the dispersion parameter.

Veranstalter
Universität Potsdam
Humboldt-Universität zu Berlin
WIAS Berlin
Dienstag, 21.05.2019, 15:00 Uhr (WIAS-ESH)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Fabian Telschow, University of California San Diego, USA:
Estimation of expected Euler characteristics of non-stationary Gaussian random fields
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Weitere Informationen
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics

Veranstalter
WIAS Berlin
Dienstag, 21.05.2019, 10:15 Uhr (WIAS-ESH)
Seminar Nichtlineare Optimierung und Inverse Probleme
Dr. Olaf Klein, WIAS Berlin:
On uncertainty quantification for models involving hysteresis effects
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
To describe hysteresis effects like magnetization or elasto-plastic behavior models involving hysteresis operators are used. The parameter within these operators are subject to uncertainties, since they have to be identified from measurements. To quantify the influence of these uncertainties, the parameter in the hysteresis operator are considered as functions of random variables. This allows to apply the methods of emphUncertainty Quantification (UQ) to the considered models. Performing UQ for models involving hysteresis effects, we get new random variables describing the uncertain outputs of the and we can compute stochastic properties of the output of the model. As example, forward UQ for a play operator with uncertain yield limits is shown. Moreover, starting from measured data for an Terfenol actuator, the identification of the parameters and their uncertainty within the hysteresis operator in the corresponding model is discussed.

Veranstalter
WIAS Berlin
Freitag, 17.05.2019, 14:00 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Prof. Peter Berg, University of Alberta , Kanada:
Energy conversion in electrokinetic flow through charged and viscoelastic nanochannels
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
The flow of ions and water through viscoelastic, nanoscopic domains forms the basis for many processes in biological materials. Surprisingly, such systems have rarely been explored for nanofluidic transport in artificial channels of technological applications such as energy harvesting, water desalination or DNA purification.
This talk explores the nonlinear coupling between wall deformation and quasi 1-D electrokinetic transport in a nanochannel with charged walls. Within the framework of nonequilibrium thermodynamics, formulae are derived for the electrokinetic transport parameters in terms of Onsager phenomenological coefficients and, subsequently, for energy conversion efficiencies. Results confirm that Onsager's reciprocity principle holds for rigid channels but breaks down in the 1-D formulation when the channel is deformed due to the introduction of a ”fictitious” diffusion term of counter-ions. Furthermore, the model predicts a reduced efficiency of electrokinetic energy harvesting for channels with soft, deformable walls.
This research is conducted in collaboration with Michael Eikerling and Mpumelelo Matse.

Veranstalter
WIAS Berlin
Donnerstag, 16.05.2019, 14:00 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Prof. Robert Eisenberg, Rush University Chicago, USA:
Voltage sensors of biological channels are nanomachines that perfectly conserve current, as Maxwell defined it
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Biological channels produce the signals of the nervous system, and coordinate the contraction of muscle, including the heart, by responding to voltage. Biological channels are proteins with a specific piece of machinery that responds to voltage, called the voltage sensor. The voltage sensor moves charges through an electric field creating a polarization (i.e., dielectric) current that can be measured in the far field some 1e23 atoms away from the channel because Maxwell's equations enforce the perfect conservation of current, as Maxwell defined it. Maxwell's current includes the polarization of the vacuum, independent of the properties of matter (including proteins!) no matter how complex. We have built a precise electromechanical model of the voltage sensor based on its atomic scale structure that fits a wide range of experimental data.

Veranstalter
WIAS Berlin
Mittwoch, 15.05.2019, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Dr. Robert Eisenberg, Rush University Chicago:
Ionic selectivity in channels: complex biology created by the balance of simple physical forces
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Mittwoch, 15.05.2019, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Frank Konietschke, Charité Berlin:
Small data: A big data problem
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Small sample sizes occur frequently and especially in preclinical research. Most statistical methods are only valid if sample sizes are large and thus, investigating the methods' behavior when samples are small is tempting. It turns out, that few statistical methods are as reliable as throwing a coin when samples are small. In this talk, we propose few improvements using resampling and permutation methods. In particular, we will answer the question "When and how do permutation methods work?". Real data sets illustrates the application of the proposed mean based and purely nonparametric rank-based methods.

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Dienstag, 14.05.2019, 15:00 Uhr (WIAS-405-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Michele Coghi, WIAS Berlin:
Rough nonlinear Fokker-Planck equations
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Abstrakt
Motivated by McKean-Vlasov diffusions with "common" noise, we present well-posedness of a nonlinear Fokker-Planck equation driven by a rough path. Joint work with Torstein Nilssen.

Veranstalter
WIAS Berlin
Dienstag, 14.05.2019, 13:30 Uhr (WIAS-406)
Seminar Materialmodellierung
Dr. Mirjam Walloth, Technische Universiät Darmstadt:
Reliable, efficient and robust a posteriori estimators for the variational inequality in fracture phase-field models
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Veranstalter
WIAS Berlin
Montag, 13.05.2019, 14:00 Uhr (WIAS-ESH)
Seminar Quantitative Biomedizin
Prof. Robert Eisenberg, Rush University Chicago, USA:
The lens of the eye: Bidomain model of an osmotic pump
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
The lens of the eye has no blood vessels to interfere with vision. The lens is far too large for diffusion to provide food and clear wastes. Experimental, theoretical and computational work has shown that the lens supports its own microcirculation. It is an osmotic pump. We introduce a general non-electro-neutral model that describes the steady state relationships among ion fluxes, water flow and electric field inside cells, and in the narrow extracellular spaces within the lens. Using asymptotic analysis, we derive a simplified model based on physiological data. The model reduces to the first generation analytical models. The full model explains additional data of some significance.

Veranstalter
WIAS Berlin
Donnerstag, 09.05.2019, 16:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Modelle der Photonik
Sebastian Franke, Technische Universität Berlin:
Quantization of dissipative resonators using quasinormal modes
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Donnerstag, 09.05.2019, 14:00 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Dr. Christoph Freysoldt, Max-Planck-Institut für Eisenforschung GmbH:
Concepts and algorithms in SPHInX
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
SPHInX is a C++ class library for computer simulations in material science, with a focus on electronic-structure theory. Our aim is to provide a framework that is on one hand enables physicists to implement easily new algorithms, and on the other hand makes use of developments in computer hardware and software design principles to achieve efficiency. I will give an overview over the hierarchy of available classes, and exemplify how mathematical and physical concepts are mapped onto C++ classes. I will then demonstrate the mutual benefit of concept and algorithm evolution. Last, some seemingly minor features of the SPHInX package will be highlighted, such as debugging checks, clocks, file parsing, or command line options, that proved to be immensely productive in daily work.

Veranstalter
WIAS Berlin
Mittwoch, 08.05.2019, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Amal Alphonse, WIAS Berlin:
Well posedness for a coupled bulk-surface system on a moving domain modelling ligand-receptor dynamics
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
This talk is on the well posedness and regularity of solutions of a reaction-diffusion system of coupled bulk-surface equations on a moving domain modelling receptor-ligand dynamics in cells. The coupling of the unknowns is through a nonlinear Robin boundary condition for the bulk quantity and the source terms for the two surface quantities. After motivating the model, I will discuss the difficulties posed by this system and the technical tools necessary in order to achieve the desired analytical results. This talk is based on a recent joint paper with Charlie Elliott (University of Warwick, UK) and Joana Terra (Universidad Nacional de Cordoba, Argentina).

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Dienstag, 07.05.2019, 13:30 Uhr (WIAS-406)
Seminar Materialmodellierung
Dr.-Ing. Rainer Falkenberg, Bundesanstalt für Materialforschung und -prüfung:
Aspects on the modelling of material degradation
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
Material degradation describes the loss of nominal strength. The physical causes as well as the consequences are often manifold: Mechanical loads exceeding a threshold value or temperature/species-induced effects are possible and can lead e.g. to a reduced load-bearing capacity in general or to crack initiation and propagation in a local sense. The formulation and solution of this initial boundary value problem must therefore cover some crucial aspects: e.g. the fulfillment of the second law of thermodynamics by the constitutive as well as the degradation model or the consideration of the PDE system's stability loss when dealing with strict local models. Well-established models that will be discussed in the finite-element framework are the fracture-mechanics based cohesive zone model, the damage-mechanics based phase-field model and the micromechanics-based Gurson--model. Furthermore, an extension will be presented that allows for the simulation of corrosion processes.

Veranstalter
WIAS Berlin
Dienstag, 30.04.2019, 15:15 Uhr (WIAS-ESH)
Oberseminar Nonlinear Dynamics
Irina Kmit, Humboldt-Universität zu Berlin:
Classical bounded and almost periodic solutions to quasilinear first-order hyperbolic systems
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
We consider boundary value problems for quasilinear first-order one-dimensional hyperbolic systems in a strip. The boundary conditions are supposed to be of a smoothing type, in the sense that the L2-generalized solutions to the initial-boundary value problems become eventually C2-smooth for any initial L2-data. We investigate small global classical solutions and obtain the existence and uniqueness result under the condition that the evolution family generated by the linearized problem has exponential dichotomy on R. We prove that the dichotomy survives under small perturbations in the leading coefficients of the hyperbolic system. Assuming that the coefficients of the hyperbolic system are almost periodic, we prove that the bounded solution is almost periodic also.

Veranstalter
Freie Universität Berlin
WIAS Berlin
Dienstag, 30.04.2019, 15:00 Uhr (WIAS-405-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Melina Freitag, University of Bath, UK:
A low-rank approach to the solution of weak constraint variational data assimilation problems
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Veranstalter
WIAS Berlin
Dienstag, 30.04.2019, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Modelle der Photonik
Prof. Deb Kane, Macquarie University Sydney, Australia:
Experimental nonlinear semiconductor-laser-based systems: Rich resources for applications and nonlinear science
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Nonlinear laser systems provide rich opportunities for both theoretical modelling and for technological applications. There is significant diversity in the dynamics of the output power as a function of time, that can be derived from a single nonlinear laser system for different operating conditions, or, across different nonlinear laser systems. We have been undertaking experiments that generate high density, and/or long time series, datasets from various nonlinear laser systems as a resource for facilitating comparisons between theory and experiment, and for answering science questions. Some of our recent work will be discussed including (i) dynamical mapping of semiconductor lasers with delayed optical feedback systems, both integrated multi-section systems and those using free space propagation; (ii) the dynamical diversity of the output of a long cavity VECSEL system primarily targeted for low pulse repetition rate passive mode-locking; and (iii) forecasting the impact of partial coherence of delayed feedback light in systems.

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Mittwoch, 24.04.2019, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Dirk Peschka, WIAS Berlin:
Modeling, simulation, and experiments for flows of concentrated suspensions
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Mittwoch, 24.04.2019, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Dr. Chen Huang, Universität St. Gallen, Schweiz:
LASSO in time and space
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
We consider the estimation and inference in a system of high-dimensional regression equations allowing for temporal and cross-sectional dependency in covariates and error processes, covering rather general forms of weak dependence. A sequence of regressions with many regressors using LASSO (Least Absolute Shrinkage and Selection Operator) is applied for variable selection purpose, and an overall penalty level is carefully chosen by a block multiplier bootstrap procedure to account for multiplicity of the equations and dependencies in the data. Correspondingly, oracle properties with a jointly selected tuning parameter are derived. We further provide high-quality de-biased simultaneous inference on the many target parameters of the system. We provide bootstrap consistency results of the test procedure, which are based on a general Bahadur representation for the Z-estimators with dependent data. Simulations demonstrate good performance of the proposed inference procedure. Finally, we apply the method to quantify spillover effects of textual sentiment indices in a financial market and to test the connectedness among sectors.

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Dienstag, 23.04.2019, 13:30 Uhr (WIAS-406)
Seminar Materialmodellierung
Marijo Milicevic, Albert-Ludwigs-Universität Freiburg:
The alternating direction method of multipliers with variable step sizes for the iterative solution of nonsmooth minimization problems and application to BV-damage evolution
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
The alternating direction method of multipliers (ADMM) is a flexible numerical method to solve a large class of convex minimization problems. Its most significant properties are the unconditional convergence with respect to the involved step size and the direct applicability. However, the performance critically depends on the choice of the step size. We propose an automated step size adjustment that relies on the monotonicity of the residual to accelerate the ADMM. Numerical experiments show a remarkable improvement over the standard ADMM with fixed step sizes. The ADMM with variable step sizes is then applied to a model for rate-independent, total variation regularized damage processes. The total variation regularization of the damage variable leads to sharp transitions of damaged to undamaged areas in the material. The results are compared to an H1-regularization of the damage and the simulations reveal that, indeed, for the total variation regularization sharp transitions can be observed whereas for the H1-regularization the interface is smeared out.

Veranstalter
WIAS Berlin
Mittwoch, 17.04.2019, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Grigor Nika, WIAS Berlin:
Multiscale modelling of magnetorheological suspensions
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Dienstag, 16.04.2019, 15:15 Uhr (WIAS-ESH)
Oberseminar Nonlinear Dynamics
Dr. Oleh Omelchenko, Universität Potsdam:
Traveling chimera states
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
We overview recent results about the traveling chimera states observed in a ring of nonlocally coupled phase oscillators with broken reflection symmetry of the coupling kernel. These states manifest themselves as coherence-incoherence patterns moving along the ring. As the coupling asymmetry grows they undergo a sequence of transformations, which can be adequately explained using the continuum limit Ott-Antonsen equation. In the context of this equation the chimera states correspond to its smooth traveling wave solutions. Using the mathematical methods from the PDE and integral equations theory we carry out asymptotic analysis of these traveling waves, describe an algorithm for their numerical continuation and explore the spectrum of the corresponding linearized equation. We show that traveling chimera states can lose their stability via fold and Hopf bifurcations. Some of the Hopf bifurcations turn out to be supercritical resulting in the observation of modulated traveling chimera states.

Veranstalter
Freie Universität Berlin
WIAS Berlin
Dienstag, 16.04.2019, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Prof. Jérôme Lelong, Université Grenoble Alpes, Frankreich:
Pricing path-dependent Bermudan options using Wiener chaos expansion: An embarrassingly parallel approach
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
In this work, we propose a new policy iteration algorithm for pricing Bermudan options when the payoff process cannot be written as a function of a lifted Markov process. Our approach is based on a modification of the well-known Longstaff Schwartz algorithm, in which we basically replace the standard least square regression by a Wiener chaos expansion. Not only does it allow us to deal with a non Markovian setting, but it also breaks the bottleneck induced by the least square regression as the coefficients of the chaos expansion are given by scalar products on the L^2 space and can therefore be approximated by independent Monte Carlo computations. This key feature enables us to provide an embarrassingly parallel algorithm.

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Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics

Veranstalter
WIAS Berlin
Dienstag, 16.04.2019, 13:30 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Dr. Li Jie, University of Cambridge, GB:
Macroscopic model for head-on binary droplet collisions in a gaseous medium
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
In this work, coalescence-bouncing transitions of head-on binary droplet collisions are predicted by a novel macroscopic model based entirely on fundamental laws of physics. By making use of an existing lubrication theory, we have modified the Navier-Stokes equations to accurately account for the rarefied nature of the interdroplet gas film. Through the disjoint pressure model, we have incorporated the intermolecular Van der Waals forces. Our model does not use any adjustable (empirical) parameters. It therefore encompasses an extreme range of length scales (more than 5 orders of magnitude): from those of the external flow in excess of the droplet size (a few hundred micros) to the effective range of the Van der Waals force around 10 nm. A state of the art moving adaptive mesh method, capable of resolving all the relevant length scales, has been employed. Our numerical simulations are able to capture the coalescence-bouncing and bouncing-coalescence transitions that are observed as the collision intensity increases. The predicted transition Weber numbers for tetradecane and water droplet collisions at different pressures show remarkably good agreement with published experimental values. Our study also sheds new light on the roles of gas density, droplet size and mean free path in the rupture of the gas film.

Veranstalter
WIAS Berlin
Donnerstag, 28.03.2019, 16:00 Uhr (WIAS-ESH)
Seminar Laserdynamik
Christian Schelte, University of the Balearic Islands, Spain:
Bistability and solitons in an injected time-delayed Kerr microcavity
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Weitere Informationen
Seminar Laserdynamik

Veranstalter
WIAS Berlin
Donnerstag, 28.03.2019, 10:00 Uhr (WIAS-406)
Seminar Laserdynamik
Aida Hajizadeh, Leibniz-Institut für Neurobiologie Magdeburg:
Computational modelling of signal processing in the human auditory cortex: Analytical solutions based on cortical network structure and oscillatory dynamics
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Weitere Informationen
Seminar Laserdynamik

Veranstalter
WIAS Berlin
Mittwoch, 27.03.2019, 10:30 Uhr (WIAS-406)
Seminar Interacting Random Systems
Dr. Giovanni Luca Torrisi, Istituto per le Applicazioni del Calcolo “Mauro Picone” (C.N.R), Italien:
The Clark-Ocone formula for point processes
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
Clark-Ocone formulas are powerful results in stochastic analysis with a variety of applications. In the talk we provide the Clark-Ocone formula for square-integrable functionals of point processes with stochastic intensity. Then we present two applications of the formula: the Poincare' inequality and a deviation bound for those functionals. Our results generalize the corresponding ones on the Poisson space

Weitere Informationen
Seminar Interacting Random Systems

Veranstalter
WIAS Berlin
Dienstag, 26.03.2019, 15:00 Uhr (WIAS-ESH)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Valeriy Avanesov, WIAS Berlin:
Nonparametric change point detection in regression
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
20. – 22. März 2019 (IAP)
Workshop/Konferenz: Leibniz MMS Days 2019
mehr ... Veranstaltungsort
Leibniz-Institute of Atmospheric Physics, Schlossstr. 6, 04318 Kühlungsborn, Germany

Veranstalter
WIAS Berlin
Dienstag, 19.03.2019, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Darina Dvinskikh, WIAS Berlin:
Distributed decentralized (stochastic) optimization for dual friendly functions
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
We study the optimal complexity bounds of first-order methods for the problem of minimizing the finite sum of convex (strongly convex) functions. To address this problem, we appeal to distributed optimization and consider the network of multiple agents (processors) which cooperatively minimize the target function. We compare two different approaches toward the finding of the minimum depending on the functions properties in terms of oracle calls and communication rounds: primal approach and dual approach, based on the assumption of existence of the Fenchel-Legendre transform of the target function.

Veranstalter
WIAS Berlin
18. – 22. März 2019 (ZIB-SEM)
Workshop/Konferenz: New Directions in Stochastic Analysis: Rough Paths, SPDEs and Related Topics
mehr ... Veranstaltungsort
Zuse-Zentrum für Informationstechnik (ZIB), Takustr. 7, 14195 Berlin, Seminarraum

Veranstalter
Technische Universität Berlin
WIAS Berlin
Universität Potsdam
Freie Universität Berlin
DFG Research Unit 2402
European Research Council
Dienstag, 12.03.2019, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Malte Weber, Humboldt-Universität zu Berlin:
Community detection using belief propagation
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
Using the message passing algorithm "belief propagation" to detect communities in a network graph, i.e. finding subsets of the nodes of the graph which are more densly connected within, than they are to the other nodes of the graph.

Veranstalter
WIAS Berlin
Donnerstag, 28.02.2019, 10:00 Uhr (WIAS-406)
Seminar Materialmodellierung
Prof. Uwe Thiele, Westfälische Wilhelms-Universität Münster:
Gradient dynamics models for films of complex fluids and beyond - dewetting, line deposition and biofilms
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
After briefly reviewing a number of experiments on dewetting and evaporating thin films/drops of simple and complex liquids, I introduce the concept of a gradient dynamics description of the evolution of interface-dominated films and drops on solid substrates. First, the case of films/drops of simple non-volatile liquid is discussed, and illustrated with results on droplet patterns and sliding droplets. As a further example, the diffusion equation is formulated as a gradient dynamics. The obtained elements are combined into a thermodynamically consistent gradient dynamics formulation for films of mixtures and surfactant suspensions [1,2]. Next, such models are employed to investigate the out-of-equilibrium process of the deposition of line patterns at receding contact lines for evaporatively dewetting solutions/suspensions [3] and in Langmuir Blodgett transfer [4]. Finally, I discuss how to combine the presented thin-film dynamics with bioactive elements to obtain models for the osmotic spreading of biofilms growing on moist agars [5]. I conclude with a summary and outlook. [1] U. Thiele, D. Todorova, H. Lopez, Phys. Rev. Lett. 111, 117801 (2013) [2] Thiele, U.; Archer, A.Pismen, L., Phys. Rev. Fluids 1, 083903 (2016). [3] U. Thiele, Advances in Colloid and Interface Science 206, 399-413 (2014). [4] M.H. Köpf and U. Thiele, Nonlinearity 27, 2711-2734 (2014). [5] Trinschek, S.; John, K.; Lecuyer, S., Thiele, U., Phys. Rev. Lett. 119, 078003 (2017). [6] Trinschek, S.; John, K. Thiele, U., Soft Matter 14, 4464-4476 (2018)

Veranstalter
WIAS Berlin
Dienstag, 26.02.2019, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr Paolo Pigato, WIAS Berlin:
Density estimates for stocastic differential equations
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Veranstalter
WIAS Berlin
Dienstag, 19.02.2019, 10:15 Uhr (WIAS-406)
Seminar Nichtlineare Optimierung und Inverse Probleme
Dr. Jan Schwientek; Holger Berthold, Fraunhofer-Institut für Techno- und Wirtschaftsmathematik ITWM:
Numerical methods for (general) semi-infinite optimization
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
Semi-infinite programming has become a powerful modeling tool in (continuous) optimization tasks and is increasingly used in many practical problems from various fields recently. This is mainly due to the enormous further development of computer technology and numerical methods, which now make such problems tractable on standard PCs within reasonable time. The talk is divided into two parts. Firstly, we give a short introduction to (general) semi-infinite programming and survey its wide application range. We then review numerical methods from literature and present new promising approaches from ITWM. In the second part of the talk we give a detailed numerical comparison of several methods on basis of an exhaustive set of test problem, partly stemming from real-world applications. The focus of this comparison is on the performance of an ITWM solver opposing the other introduced numerical methods concerning running time, needed iterations, and accuracy. In doing so we investigate the behaviour of the implemented solvers also with respect to different mathematical environments like convexity and dimensionality.

Veranstalter
WIAS Berlin
Donnerstag, 14.02.2019, 14:30 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Philipp Schroeder, Georg-August-Universität Göttingen:
Building bridges: Pressure-robust FEM, Beltrami flows and structure preservation in incompressible CFD
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
In this talk, an attempt is made to explain why pressure-robust FEM are (by construction) superior when it comes to simulating incompressible flows with a large amount of large-scale/coherent structures. We show that flows with large-scale structures are frequently dominated by large gradient (curl-free) forces. Pressure-robust methods are designed in exactly such a way that they can treat those forces more accurately than non-pressure-robust methods. Furthermore, the class of generalised Beltrami flows is introduced and placed in context with structure preservation and pressure-robustness. Several numerical examples, both in 2D and in 3D, of laminar and turbulent flows, are shown which underline our statements. All pressure-robust computations make us of (high-order) exactly divergence-free H(div)-(H)DG methods.

Veranstalter
WIAS Berlin
Mittwoch, 13.02.2019, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Alexander Mielke, WIAS Berlin:
On similarity solutions for Kolmogorov's two-equation model for turbulence
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
Mittwoch, 13.02.2019, 11:30 Uhr (WIAS-406)
Seminar Interacting Random Systems
Peter Neijjar, Institute of Science and Technology Austria (IST Austria), Österreich:
Product limit laws at shocks in (T)ASEP
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
We consider the asymmetric simple exclusion process (ASEP) on Z with an initial data such that in the large time particle density r a discontinuity at the origin is created, where the value of r jumps from zero to one, but r (resp. 1-r) is strictly positive to the left (resp. right) of the origin. We consider the position of a particle macroscopically located at the discontinuity, and show that its limit law has a cutoff at the origin. Inside the discontinuity region, we show that a discrete product limit law arises, which bounds from above the limiting fluctuations of the particle in the general ASEP, and equals them in the totally ASEP. Sending the size of the shock region to infinity, we recover the product of GUE distributions previously observed at shocks in the totally ASEP.

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Seminar Interacting Random Systems

Veranstalter
WIAS Berlin
Mittwoch, 13.02.2019, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Professor Albert Cohen, Université Pierre et Marie Curie, Paris, France:
Optimal non-intrusive methods in high-dimension
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Motivated by non-intrusive approaches for high-dimensional parametric PDEs, we consider the approximation of an unknown arbirary function in any dimension from the data of point samples, where the approximants are picked from given or adaptively chosen finite dimensional spaces. One principal objective is to obtain an approximation which performs as good as the orthogonal projection using a sampling budget that is linear in the dimension of the approximating space. Using a particular sampling measure, this objective turns out to be met by both least-squares and pseudo-spectral methods in some probabilistic sense, however with some notable distinctions that will be discussed in this talk.

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Donnerstag, 07.02.2019, 16:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Modelle der Photonik
Prof. Raimondas Čiegis, Vilnius Gediminas Technical University, Lithuania:
Numerical simulation of models with the memory effects: Diffusion, heat conduction, nonlinear optics
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Mittwoch, 06.02.2019, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Serhiy Yanchuk, Technische Universität Berlin:
Temporal dissipative solitons in delay feedback systems
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Mittwoch, 06.02.2019, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Christian Clason, Universität Duisburg-Essen:
Generalized modes in Bayesian inverse problems
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
In this talk, we consider non-parametric modes and MAP estimates for priors that do not admit continuous densities, for which previous approaches based on small ball probabilities fail. We propose a novel de nition of generalized modes based on the concept of approximating sequences, which reduce to the classical mode in certain situations that include Gaussian priors but also exist for a more general class of priors. The latter includes the case of priors that impose strict bounds on the admissible parameters and in particular of uniform priors. For uniform priors de ned by random series with uniformly distributed coecients, we show that generalized MAP estimates but not classical MAP estimates can be characterized as minimizers of a suitable functional that plays the role of a generalized OnsagerMachlup functional. This is then used to show consistency of nonlinear Bayesian inverse problems with uniform priors and Gaussian noise. We also address the numerical computation of the corresponding generalized MAP estimates.

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Dienstag, 05.02.2019, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Jórg Polzehl, WIAS Berlin:
Towards in-vivo histology of the brain - some statistical contributions
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Veranstalter
WIAS Berlin
Mittwoch, 30.01.2019, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Barbara Zwicknagl, Technische Universität Berlin:
On variational models for (almost) stress-free martensitic inclusions
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Mittwoch, 30.01.2019, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Jean-Pierre Florens, University of Toulouse:
Is completeness necessary? Penalized estimation in non identified linear models
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Dienstag, 29.01.2019, 15:15 Uhr (FU Berlin)
Oberseminar Nonlinear Dynamics
Alejandro Kocsard, Universidade Federal Fluminense, Brazil:
Random walks, synchronization and existence of invariant measures
mehr ... Veranstaltungsort
Freie Universität Berlin, Arnimallee 3, 14195 Berlin, Hinterhaus, Raum 130

Abstrakt
In this talk we start discussing some classical results due to Furstenberg [Fur63] about random product of matrices and how a certain form of synchronization naturally appears in this context. Then we shall consider some extensions of these results for random walks on compact smooth manifolds [Led84, Ant84, Cra90]. Finally we will see how these ideas can be used to study the existence of invariant (probability) measures for smooth actions on manifolds by some groups which are a priori non-amenable.

Veranstalter
Freie Universität Berlin
WIAS Berlin
Dienstag, 29.01.2019, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Darina Dvinskikh, WIAS Berlin:
Distributed decentralized (stochastic) optimization for dual-friendly functions
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Veranstalter
WIAS Berlin
Dienstag, 29.01.2019, 13:30 Uhr (WIAS-406)
Seminar Materialmodellierung
Prof. Vittorio Romano, University of Catania, Italy:
Charge and phonon transport in graphene
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
The last years have witnessed a great interest for 2D-materials due to their promising applications. The most investigated one is graphene which is considered as a potential new material to exploit in nano-electronic and optoelectronic devices. Charge transport in graphene can be described with several degrees of physical complexity. At quantum level an accurate model is represented by the Wigner equation but in several cases its semiclassical limit, the Boltzmann equation, constitutes a fully acceptable model. However, the numerical difficulties encountered in the direct solution of both the Wigner and the semiclassical Boltzmann equation has prompted the development of hydrodynamical, energy transport and drift diffusion models, in view of the design of a future generation of electron devices where graphene replaces standard semiconductors like silicon and gallium arsenide. Moreover, thermal effects in low dimensional structures play a relevant role and, therefore, also phonon transport must be included. Interesting new mathematical issues related to the peculiar features of graphene arise. The main aspects will be discussed and recent results illustrated in the perspective of future developments, in particular the optimization of graphene field effect transistors.

Veranstalter
WIAS Berlin
Donnerstag, 24.01.2019, 16:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Modelle der Photonik
Dr. Stefan Schulz, Tyndall National Institute, Ireland:
Electronic, optical and transport properties of III--N alloys and heterostructures: Insights from and challenges for theoretical modelling
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
InGaN-based heterostructures have attracted considerable interest for a variety of different applications. For example, the key building blocks of modern light-emitting devices operating in the blue spectral region are c-plane InGaN/GaN quantum wells (QWs). It is remarkable that these devices are so successful, given the extremely high defect densities in InGaN/GaN systems. This defect insensitivity has been widely attributed to carrier localization effects introduced by alloy fluctuations, preventing carriers to reach non-radiative recombination centers. It is only recently that localization effects in InGaN QWs have been addressed in theoretical studies and, in comparison to other III-V materials and alloys, such such as InGaAs, several key properties are not well understood. This lack of understanding ranges from fundamental material properties up to devices related aspects. In this talk, electronic and optical properties of bulk alloys, such as AlInN or InGaN, as well as InGaN quantum wells will be discussed by means of atomistic models. These approaches range from first-principles density functional methods up to in-house developed empirical tight-binding models. Here, special attention is paid to nanoscale effects such as local fluctuations in intrinsic built-in fields. Theoretical results will be compared to available experimental data. Finally, theoretical and modeling challenges for instance in the area of transport calculations will be discussed and reviewed. Initial results of coupled tight-binding non-equilibrium Green?s function transport calculations will be presented.

Veranstalter
WIAS Berlin
Mittwoch, 23.01.2019, 15:00 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Prof. Vladimir A. Garanzha, Russian Academy of Sciences:
Moving adaptive meshes based on the hyperelastic stress deformation
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
We suggest an algorithm for the time-dependent mesh deformation based on the minimization of hyperelastic quasi-isometric functional without introducing time derivatives. The source of deformation is a time-dependent metric tensor in Eulerian coordinates. Due to the nonlinearity of the Euler-Lagrange equations we can not assume that the norm of its residual is reduced to zero at each time step. Thus, time continuity of the moving mesh is not guaranteed since iterative minimization may result in considerable displacements even for infinitely small time steps. To solve this problem, we introduce a special variant of factorized representation of Lagrangian metric tensor and nonlinear interpolation procedure for factors of this metric tensor. Continuation problem with respect to interpolation parameters is similar to the hypoelastic deformation with a controlled stress relaxation. At each time step we start with a Lagrangian metric tensor which completely eliminates internal elastic stresses and makes current mesh an exact solution of the Euler-Lagrange equations. The continuation procedure gradually introduces internal stresses back while forcing the deformation to follow the prescribed Eulerian metric tensor. At each step of the continuation procedure the functional is approximately minimized using a few steps of the parallel preconditioned gradient search algorithm. We derive an auxiliary discrete evolution equation for target shape matrices (factors of Lagrangian metric tensor) which resembles stress relaxation equations in hypoelasticity. We present 2d and 3d examples of moving deforming meshes which serve to represent moving bodies for parallel immersed boundary flow solver.

Veranstalter
WIAS Berlin
Mittwoch, 23.01.2019, 13:00 Uhr (WIAS-ESH)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Prof. Dr. Maria Soledad Aronna, Technische Universität:
On second order optimality conditions for control-affine problems: the finite and infinite dimensional case
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
In this talk I will present the main features of first and second order optimality conditions for optimal control problems of ordinary differential equations that are affine with respect to the control. Problems will be considered in the presence of control constraints. Both necessary and sufficient conditions for optimality will be presented. I will then show the extension of those results for a class of bilinear partial differential equations. Finally, if time allows it, I will briefly discuss the state-constrained case.

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
Mittwoch, 23.01.2019, 11:30 Uhr (WIAS-406)
Seminar Interacting Random Systems
Pierre Houdebert, Aix-Marseille Université, Frankreich:
Sharp phase transition for the Widom-Rowlinson model
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
The Widom-Rowlinson model is formally defined as two homogeneous Poisson point processes forbidding the points of different type to be too close. For this Gibbs model the question of uniqueness/ non-uniqueness depending on the two intensities is relevant. This model is famous because it was the first continuum Gibbs model for which phase transition was proven, in the symmetric case of equal intensities large enough. But nothing was known in the non-symmetric case, where it is conjectured that uniqueness would hold. In a recent work with D. Dereudre (Lille), we partially solved this conjecture, proving that for large enough activities the phase transition is only possible in the symmetric case of equal intensities. The proof uses percolation and stochastic domination arguments.

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Seminar Interacting Random Systems

Veranstalter
WIAS Berlin
Dienstag, 22.01.2019, 15:15 Uhr (FU Berlin)
Oberseminar Nonlinear Dynamics
Edgard Pimentel, Pontifical Catholic University of Rio de Janeiro, Brazil:
Geometric methods in regularity theory for nonlinear PDEs
mehr ... Veranstaltungsort
Freie Universität Berlin, Arnimallee 3, 14195 Berlin, Hinterhaus, Raum 130

Abstrakt
In this talk we examine the regularity theory of the solutions to a few examples of nonlinear PDEs. Arguing through a genuinely geometrical method, we produce regularity results in Sobolev and Hölder spaces, including some borderline cases. Our techniques relate a problem of interest to a further one - for which a richer theory is available - by means of a geometric structure, e..g., a path. Ideally, information is transported along such a path, giving access to finer properties of the original equation. In the first part of the talk, we cover examples including elliptic and parabolic fully nonlinear problems, the Isaacs equation and a double divergence model. Then we proceed to the setting of state-dependent degenerate problems and report recent (optimal) results. We close the talk with a discussion on open problems and further directions of work.

Veranstalter
Freie Universität Berlin
WIAS Berlin
Dienstag, 22.01.2019, 15:00 Uhr (WIAS-ESH)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Prof. Ahmed Kebaier, Université Paris 13, France:
Asymptotic properties of maximum likelihood estimator for the drift parameters of jump-type square root processes
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

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Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics

Veranstalter
WIAS Berlin
Mittwoch, 16.01.2019, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Joachim Rehberg, WIAS Berlin:
An extrapolation for the Lax-Milgram isomorphism for second order divergence operators
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
In 1989 Gröger generalized the famous isomorphy result of Meyer to a setting which allows for mixed boundary conditions. We again generalize Gröger's result to much more general geometric configurations.

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Mittwoch, 16.01.2019, 13:00 Uhr (WIAS-ESH)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Tobias Keil, WIAS Berlin:
Optimal control of a coupled Cahn--Hilliard--Navier--Stokes system with variable fluid densities
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
This talk is concerned with the optimal control of two immiscible fluids with non-matched densities. For the mathematical formulation of the fluid phases, we use a coupled Cahn-Hilliard/Navier-Stokes system by Abels, Garcke and Grün, which involves a variational inequality of fourth order. We verify the existence of solutions to a suitable time discretization of the system and formulate an associated optimal control problem. We further discuss the differentiability properties of the control-to-state operator and the corresponding stationarity concepts and present strong stationarity conditions for the problem. This enables us to provide a numerical solution algorithm which terminates at an at least C-stationary point which - in the best case - is even strongly stationary. The method is based on an adaptation of a bundle-free implicit programming approach for MPECs in function space presented by Hintermüller and Surowiec in 2016.

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Joint Research Seminar on Mathematical Optimization / Non-smooth Variational Problems and Operator Equations

Veranstalter
WIAS Berlin
Mittwoch, 16.01.2019, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Dr. Mathias Trabs, Universität Hamburg:
Parameter estimation for stochastic PDEs based on discrete observations in time and space
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Motivated by random phenomena in natural science as well as by mathematical finance, stochastic partial differential equations (SPDEs) have been intensively studied during the last fifty years with a main focus on theoretical analytic and probabilistic aspects. Thanks to the exploding number of available data and the fast progress in information technology, SPDE models become nowadays increasingly popular for practitioners, for instance, to model neuronal systems or interest rate fluctuations to give only two examples. Consequently, statistical methods are required to calibrate this class of complex models. We study the parameter estimation for parabolic, linear, second order SPDEs observing a mild solution on a discrete grid in time and space. Focusing first on volatility estimation and assuming a high-frequency regime in time, we provide an explicit and easy to implement method of moments estimator based on squared time increments of the process. If the observation frequency in time is finer than in space, the estimator is consistent and admits a central limit theorem. This is established moreover for the estimation of the integrated volatility in a semi-parametric framework. In a second step, we consider not only time increments of the solution field but also space increments as well as space-time increments. This allows for the construction of estimators which are robust with respect to the sampling regime, i.e., they are also applicable if the observation grid in space is finer than in time. Finally, we discuss the estimation of the parameters in the differential operator which determines the SPDE. This talk is based on joint works with Markus Bibinger and Florian Hildebrandt.

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Dienstag, 15.01.2019, 15:15 Uhr (FU Berlin)
Oberseminar Nonlinear Dynamics
Prof. Willie Hsia, National Taiwan University:
On the mathematical analysis of the synchronization theory with time-delay effect
mehr ... Veranstaltungsort
Freie Universität Berlin, Arnimallee 3, 14195 Berlin, Hinterhaus, Raum 130

Abstrakt
In this lecture, we will introduce a newly developed mathematical theory on the synchronized collective behavior of the Kuramoto oscillators with time-delayed interactions and phase lag effect. Both the phase synchronization and frequency synchronization are in view. This is joint work with Bongsuk Kwon, Chang-Yeol Jung and Yoshihiro Ueda.

Veranstalter
Freie Universität Berlin
WIAS Berlin
Dienstag, 15.01.2019, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Alexander Gasnikov, Moscow Institute of Physics and Technology, Russische Föderation:
Adaptive accelerated stochastic gradient descent
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

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Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics

Veranstalter
WIAS Berlin
Freitag, 11.01.2019, 15:00 Uhr (WIAS-406)
Seminar Interacting Random Systems
Dr. Darcy Camargo, The Weizmann Institute of Science, Israel:
The meteor process stationary distribution
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
The Potlatch and Smoothing processes have a long story of studies and application, but even today not much is known about their stationary distributions when they exist. In this talk we will present the simplest version of such models and expose recent developments and techniques used to get them, especially the coupling by WIMPs and Matching.

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Seminar Interacting Random Systems

Veranstalter
WIAS Berlin
Donnerstag, 10.01.2019, 14:00 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Dr. Naveed Ahmed, Lahore University of Management Sciences, Pakistan:
Numerical comparisons of finite element stabilized methods for a 2D vortex dynamics simulation at high Reynolds number
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
In this talk, I will present an up-to-date and classical Finite Element (FE) stabiliz ed methods for time-dependent incompressible flows. All studied methods belong to the Variational MultiScale (VMS) framework. So, different realizations of stabilized FE-VMS methods are compared in a high Reynolds number vortex dynamics simulation. In particular, a fully Residual-Based (RB)-VMS method is compared with the classical Streamline-Upwind Petrov--Galerkin (SUPG) method together with grad-div stabilization, a standard one-level Local Projection Stabilization (LPS) method, and a recently proposed LPS method by interpolation. These procedures do not make use of the statistical theory of equilibrium turbulence, and no ad-hoc eddy viscosity modeling is required for all methods. Applications to the simulation of a high Reynolds numbers flow with vortical structures on relatively coarse grids are showcased, by focusing on a two-dimensional plane mixing-layer flow. Both Inf-Sup Stable (ISS) and Equal Order (EO) (H^1)-conforming FE pairs are explored, using a second-order semi-implicit Backward Differentiation Formula (BDF2) in time. Based on the numerical studies, it is concluded that the SUPG method using EO FE pairs performs best among all methods. Furthermore, there seems to be no reason to extend the SUPG method by the higher order terms of the RB-VMS method.

Veranstalter
WIAS Berlin
Mittwoch, 09.01.2019, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar) & Seminar Materialmodellierung
Prof. Dr. Peter Friz, TU Berlin/ WIAS Berlin:
On differential equations with singular forcing
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
There is much recent progress in our ability to analyze (ordinary and partial) differential equations subjected to singular forcing. This will not be a big survey talk, I will instead focus on some explicit examples.

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Mittwoch, 09.01.2019, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Dr. Frank Werner, Georg-August-Universität Göttingen:
Empirical risk minimization as parameter choice rule for filter-based regularization methods
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
We consider a-posteriori parameter choice rules for filter based linear regularization methods in the statistical inverse problem setting. In particular, we investigate the choice of the regularization parameter by minimizing an unbiased estimate of the predictive risk. This parameter choice rule and its usage are well-known in the literature, but oracle inequalities and optimality results in this general setting are unknown. We prove a (generalized) oracle inequality, which relates the direct risk with the minimal prediction risk. From this oracle inequality, we are then able to conclude that filter based regularization methods with the investigated parameter choice rule achieve convergence rates of optimal order with respect to the mean integrated squared error in mildly ill-posed problems. Finally, we also present numerical simulations, which support the order optimality of the method and the finite sample performance of the parameter choice. In these simulations, we also investigate the behavior of different a posteriori parameter choice methods in exponentially ill-posed problems. This is joint work with Housen Li (University of Göttingen).

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Mittwoch, 19.12.2018, 11:30 Uhr (WIAS-406)
Seminar Interacting Random Systems
Carina Betken, Universität Osnabrück:
Dynamic random graphs: Sedentary random waypoint and preferential attachment models
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

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Seminar Interacting Random Systems

Veranstalter
WIAS Berlin
Mittwoch, 19.12.2018, 11:00 Uhr (WIAS-ESH)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Dr. Olivier Huber, Humboldt-Universität zu Berlin:
Using reformulations in nonsmooth mathematical programming: the examples of friction contact problems and optimal value functions
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
In this talk we illustrate how reformulations can be used to deal with nonsmoothness. In particular, we look at two specific structures arising from applications: the first one concerns finite dimensional variational inequalities with a nonsmooth functional. This is motivated by the study of contact problems with Coulomb friction. The second example involves optimization models with Optimal Value Functions (OVF) in the problem data. The OVF concept subsumes regularizers from fitting problems and coherent risk measures from Stochastic Optimization. Each reformulation scheme rely on tools from convex and variational analysis. In both instances, we obtain equivalent problems that are solvable by off-the-shelf solvers. Finally, numerical results are presented and discussed: in the OVF case, the example is a risk-averse equilibrium problem from the electricity market.

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Joint Research Seminar on Mathematical Optimization / Non-smooth Variational Problems and Operator Equations

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
Mittwoch, 12.12.2018, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Joachim Rehberg, WIAS Berlin:
L bounds for the Neumann problem
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

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Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
Mittwoch, 12.12.2018, 11:30 Uhr (WIAS-406)
Seminar Interacting Random Systems
Prof. Dr. Kwabena Doku-Amponsah, University of Ghana School of Physical and Mathematical Sciences, Ghana, Republik:
Joint large deviation result for empirical measures of the random geometric graphs
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
We prove joint large deviation principle for the empirical pair measure and empirical locality measure of the near intermediate coloured random geometric graph models on n points picked uniformly in the space [0; 1]d; for d 2 N: From this result we obtain large deviation principles for the number of edges per vertex, the degree distribution and the proportion of isolated vertices for the near intermediate random geometric graph models.

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Seminar Interacting Random Systems

Veranstalter
WIAS Berlin
Mittwoch, 12.12.2018, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Botond Szabo, Universität Leiden, Niederlande:
On the fundamental understanding of distributed computation
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
In recent years, the amount of available information has become so vast in certain fields of applications that it is infeasible or undesirable to carry out the computations on a single server. This has motivated the design and study of distributed statistical or learning methods. In distributed methods, the data is split amongst different administrative units and computations are carried out locally in parallel to each other. The outcome of the local computations are then aggregated into a final result on a central machine. In this talk we will consider the limitations and guarantees of distributed methods in general under communication constraints (i.e. only limited amount of bits are allowed to be transmitted between the machines) on the random design regression model. We derive minimax lower bounds, matching upper bounds and provide adaptive estimators reaching these limits. This is an ongoing joint work with Harry van Zanten.

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Westfälische Wilhelms-Universität Münster
Dienstag, 11.12.2018, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Prof. Mathias Staudigl, Universität Maastricht, Niederlande:
On the convergence of gradient-like flows with noisy gradient input
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Veranstalter
WIAS Berlin
Dienstag, 11.12.2018, 13:30 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Dr. Wolfgang Dreyer, WIAS Berlin:
Kinetic theory of incompressible non-newtonian fluids
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
We consider a non-newtonian fluid consisting of a newtonian solvent and polymer molecules as solute. The kinetic theory models the polymer molecules as dumbbells and compiles dynamical equations that embody internal dumbbell interactions and interactions of the dumbbell with the surrounding fluid via its velocity gradient. The dynamical equations imply a generalized Fokker-Planck equation for a distribution function of the system of polymer molecules. Then the Fokker--Planck equation is used to derive macroscopic evolution equations for certain mean values. For given velocity gradient of the fluid, three evolution equations are needed to end up with a closed equation system for the polymer molecules. The corresponding three thermodynamic variables are the number density, the mean squared end-to end distance of a polymer molecule and the polymer stress. Finally we prove an H-theorem and solve the subtle problem of thermodynamic consistency.

Veranstalter
WIAS Berlin
Dienstag, 11.12.2018, 10:15 Uhr (WIAS-406)
Seminar Nichtlineare Optimierung und Inverse Probleme
Dr. Jitka Machalová, Palacky University Olomouc, Tschechische Republik:
Contact problem solution for nonlinear beam and foundation by CVM
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Veranstalter
WIAS Berlin
5. – 7. Dezember 2018 (WIAS-ESH)
Workshop/Konferenz: 13th International Workshop on Variational Multiscale and Stabilized Finite Elements
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Mittwoch, 05.12.2018, 11:30 Uhr (WIAS-HVP-3.13)
Seminar Interacting Random Systems
Prof. Dr. Serguei Popov, University of Campinas --- UNICAMP, Brasilien:
Two-dimensional random interlacements, conditional SRW, and the cryptocurrencies
mehr ... Veranstaltungsort
Weierstraß-Institut, Hausvogteiplatz 11A, 10117 Berlin, 3. Etage, Raum: 3.13

Abstrakt
Serguei Popov will briefly describe his recent research topics mentioned in the title.

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Seminar Interacting Random Systems

Veranstalter
WIAS Berlin
Mittwoch, 05.12.2018, 10:00 Uhr (WIAS-405-406)
Forschungsseminar Mathematische Statistik
Matthias Löffler, University of Cambridge, GB:
Spectral thresholding for Markov chain transition operators
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Abstrakt
We consider estimation of the transition operator P of a Markov chain and its transition density p where the eigenvalues of P are assumed to decay exponentially fast. This is for instance the case for periodised multi-dimensional diffusions observed in low frequency. We investigate the performance of a spectral hard thresholded Galerkin-type estimator for P and p, discarding most of the estimated eigenpairs. Our main contribution is that we show its statistical optimality by establishing matching minimax upper and lower bounds in L^2-loss. Particularly, the effect of the dimension d on the nonparametric rate improves from 2d to d.

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Forschungsseminar “Mathematische Statistik”

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Dienstag, 04.12.2018, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Yangwen Sun, Humboldt-Universität zu Berlin:
Complete graph based online change point detection
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

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Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics

Veranstalter
WIAS Berlin
Freitag, 30.11.2018, 13:00 Uhr (WIAS-ESH)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Steven-Marian Stengl:
Uncertainty Quantification of the Ambrosio--Tortorelli approximation in image segmentation
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
In this talk we want to deal with quantification of uncertainties in image segmentation based on the Mumford--Shah model. The aim is to address the error propagation of noise and other error types in the original image to the segmentation and especially the edges. We analyze therefore the in the literature well-known Ambrosio--Tortorelli approximation and discuss the existence of measurable selections of its solutions as well as sampling-based methods. We end the talk with numerical examples.

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Joint Research Seminar on Mathematical Optimization / Non-smooth Variational Problems and Operator Equations

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Donnerstag, 29.11.2018, 14:00 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Prof. J.H.M. ten Thije Boonkkamp, Eindhoven University of Technology, Niederlande:
Flux vector approximation schemes for systems of conservation laws
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Conservation laws in continuum physics are often coupled, for example the continuity equations for a reacting gas mixture or a plasma are coupled through multi-species diffusion and a complicated reaction mechanism. For space discretisation of these equations we employ the finite volume method. The purpose of this talk is to present novel flux vector approximation schemes that incorporate this coupling in the discretisation. More specifically, we consider as model problems linear advection-diffusion systems with a nonlinear source and linear diffusion-reaction systems, also with a nonlinear source. The new flux approximation schemes are inspired by the complete flux scheme for scalar equations, see [1]. An extension to systems of equations is presented in [2]. The basic idea is to compute the numerical flux vector at a cell interface from a local inhomogeneous ODE-system, thus including the nonlinear source. As a consequence, the numerical flux vector is the superposition of a homogeneous flux, corresponding to the homogeneous ODE-system, and an inhomogeneous flux, taking into account the effect of the nonlinear source. The homogeneous ODE-system is either an advection-diffusion system or a diffusion-reaction system. In the first case, the homogeneous flux contains only real-valued exponentials, on the other hand, in the second case, also complex-valued components are possible, generating oscillatory solutions. The inclusion of the inhomogeneous flux makes that all schemes display second order convergence, uniformly in all parameters (Peclet and Damköhler numbers). The performance of the novel schemes is demonstrated for several test cases, moreover, we investigate several limiting cases.
References
[1] J.H.M. ten Thije Boonkkamp and M.J.H. Anthonissen, “The finite volume-complete flux scheme for advection-diffusion-reaction equations”, J. Sci. Comput., 46, 47--70, (2011).
[2] J.H.M. ten Thije Boonkkamp, J. van Dijk, L. Liu and K.S.C. Peerenboom, “Extension of the complete flux scheme to systems of comservation laws”, J. Sci. Comput., 53, 552--568, (2012).

Veranstalter
WIAS Berlin
Mittwoch, 28.11.2018, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Pierre-Étienne Druet, WIAS:
Analysis of mass transfer, Navier--Stokes equations for multicomponent fluids subject to a volume constraint
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Mittwoch, 28.11.2018, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Melanie Schienle, Karlsruher Institut für Technologie:
Determination of vector error correction models for different types of high-dimensionality
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
We provide a shrinkage type methodology which allows for simultaneous model selection and estimation of vector error correction models (VECM) when the dimension is large and can increase with sample size. Model determination is treated as a joint selection problem of cointegrating rank and autoregressive lags under respective practically valid sparsity assumptions. We show consistency of the selection mechanism by the resulting Lasso-VECM estimator under very general assumptions on dimension, rank and error terms. Moreover, with computational complexity of a linear programming problem only, the procedure remains computationally tractable in high dimensions. For the subcase of finite dimensions, we can modify and tailor the procedure with elementwise selection consistency. For ultra-high dimensions we suggest a completely diferent pre-screening approach. We demonstrate the effectiveness of the proposed techniques in simulations and an empirical application to recent CDS data after the financial crisis.

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Forschungsseminar “Mathematische Statistik”

Veranstalter
Universität Potsdam
Humboldt-Universität zu Berlin
WIAS Berlin
Dienstag, 27.11.2018, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Yegor Klochkov, Humboldt-Universitát zu Berlin:
On uniform Hanson-Wright type inequalities for sub-Gaussian entries
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
We investigate concentration of supremum of quadratic forms via entropy inequality. The problem is well understood for Gaussian vectors, and, more generally for K-concentrated vectors. We extend the result to vectors with indpenent sub-Gaussian entries with an extra logarythmic term. This is a joint work with Nikita Zhivotovskiy.

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Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics

Veranstalter
WIAS Berlin
Mittwoch, 21.11.2018, 10:45 Uhr (WIAS-HVP-3.13)
Seminar Interacting Random Systems
Wen Sun, TU Berlin:
On the asymptotic distribution of nucleation times of polymerization processes
mehr ... Veranstaltungsort
Weierstraß-Institut, Hausvogteiplatz 11A, 10117 Berlin, 3. Etage, Raum: 3.13

Abstrakt
Experiments have shown that there is a sharp phase transition in polymerization: it takes long time to have small amount of stable polymers and once some amount of stable polymers appear, very quickly all particles are polymerized. Moreover, the lag time usually have a very high variance. We propose a growth-fragmentation model with a critical mass to explain these phenomenon. Particles having a mass less than this critical mass are unstable -- they are fragmented much more quicker than the larger particles. A scaling approach is used, by taking the initial total mass N as a scaling parameter and assuming that the ratio of the unstable fragmentation rates to stable fragmentation rates are of order Phi(N), which is a non-decreasing function of N. We study the time evolution of this infinite dimension process under a certain class of fragmentation distributions. We show that 1) with a proper scaling parameter, the time (T) spent for the stable polymers that generating from small particles is asymptotically exponential distributed; 2) the time for the growth of stable particles has a much smaller order than T. The exponential distribution explains the high variance and the different time scales explain the sharp phase transition. These results are proved via stochastic calculus, estimations for occupation measures on different time scales, some coupling techniques and branching processes. It is a joint work with Philippe Robert.

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Forschungsgruppenseminar Interacting Random System

Veranstalter
WIAS Berlin
Mittwoch, 21.11.2018, 10:00 Uhr (WIAS-406)
Forschungsseminar Mathematische Statistik
Prof. Markus Bibinger, Universität Marburg:
Statistical analysis of path properties of volatility
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
In this talk, we review recent contributions on statistical theory to infer path properties of volatility. The interest is in the latent volatility of an It^o semimartingale, the latter being discretely observed over a fixed time horizon. We consider tests to discriminate continuous paths from paths with volatility jumps. Both a local test for jumps at speciied times and a global test for jumps over the whole observation interval are discussed. We establish consistency and optimality properties under infill asymptotics, also for observations with additional additive noise. Recently, there is high interest in the smoothness regularity of the volatility process as confl icting models are proposed in the literature. To address this point, we consider inference on the Hurst exponent of fractional stochastic volatility processes. Even though the regularity of the volatility determines optimal spot volatility estimation methods, forecasting techniques and the volatility persistence, identifiability is an unsolved question in high-frequency statistics. We discuss a first approach which can reveal if path properties are stable over time or changing. Eventually, we discuss some recent considerations and conjectures on this open question. The related easier problem of inference on the Hurst exponent from direct discrete observations of a fractional Brownian motion is also visited.

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Dienstag, 20.11.2018, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Michele Coghi, WIAS (Berlin):
Pathwise McKean-Vlasov theory
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
We take a pathwise approach to classical McKean-Vlasov stochastic differential equations, as found e.g. in Sznitmann. We are much inspired by Cass-Lyons (and more recent works of Bailleul et al.), but avoid all rough path complications due to our focus on additive noise. The resulting “pathwise McKean-Vlasov theory” is both simple and powerful: as applications we discuss propagation of chaos with a priori independence and exchangeability assumption; common noise and reflecting boundaries are also easy to handle in this framework, last not least we can generalise the Dawson-Gärtner large deviation result to non-Brownian noise.

Veranstalter
WIAS Berlin
19. – 21. November 2018 (WIAS-ESH)
Workshop/Konferenz: Dynamics of Coupled Oscillator Systems
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Freitag, 16.11.2018, 14:00 Uhr (WIAS-ESH)
Seminar Quantitative Biomedizin
Priv.-Doz. Dr Thomas Bocklitz, Leibniz-Institut für Photonische Technologien:
Machine learning of molecular sensitive spectral and image data
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Molecular sensitive imaging modalities, like multimodal imaging, e.g. the combination of coherent-anti-Stokes Raman scattering (CARS), second-harmonic generation (SHG) and two-photon-excited fluorescence (TPEF), and Raman spectroscopic imaging feature a unique potential for disease diagnostics and treatment monitoring. In proof-of-principle studies the potential of these both imaging modalities for cancer detection and diagnostics of chronic inflammatory bowel diseases could be demonstrated. In order to utilize the huge potential of multimodal imaging and Raman spectroscopic imaging for diagnostic tasks powerful machine learning methods are necessary, which translate the experimental imaging data into medical information. This translation process is composed of image standardization, correction procedures to eliminate corrupting contributions within the acquired images, image enhancement methods and feature extraction procedures in the case of multimodal imaging. For Raman spectroscopic imaging advanced signal processing for correcting and standardizing the spectral data is necessary. After this so-called pre-processing was carried out, models like regression or classification methods can be applied to extract medical relevant information. This contribution shows, how the pre-processing have to be tailored to the measurement modalities and how the feature extraction has to be adapted to the samples and the specific diagnostic task to be solved.

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Seminar "`Quantitative Biomedizin"'

Veranstalter
WIAS Berlin
Donnerstag, 15.11.2018, 16:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Modelle der Photonik
Dr. Gediminas Slekas, Vilnius Gediminas Technical University, Lithuania:
Solution of Boltzmann transport equation for photo-excited electrons in semiconductor with strong electric field
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Photoconductive terahertz antennas based on a low temperature grown gallium arsenide are one of the most popular and most promising terahertz wave generators developed so far. The operation of such antennas is based on relaxation of photo-excited electrons in high electric field. Electrons excited to the conductivity band of semiconductor are moving towards antennas contacts producing electric current, which induces terahertz pulse. Whereas the electron recombination time is less than picosecond, the dynamics of electrons is highly non-linear. Drift-diffusion model is the simplest non-trivial carrier transport model which is derived as the zeroth order moment of Boltzmann transport equation. For many decades it has been used as a basic tool for simulation of semiconductor devices. However, non-linear effects, such as electron velocity overshoot, are not described by drift-diffusion equations, so higher order transport models are used to include this effect into calculations. On the one hand, higher order models still require a lot of fine-tuning and a detailed understanding of the underlying physical phenomena. On the other hand, they are still not adapted to the case when initial distribution of electrons is far from thermal equilibrium, which is true in the latter case. Monte Carlo simulation is the golden tool for solution of Boltzmann equation. However this method is very computationally expensive, so it is not very reasonable for optimization problems.

Veranstalter
WIAS Berlin
Donnerstag, 15.11.2018, 14:00 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Prof. Jonathan Shewchuk, University of California at Berkeley, USA:
Higher-quality tetrahedral mesh generation for domains with small angles by constrained Delaunay refinement
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Most algorithms for guaranteed-quality tetrahedral mesh generation create Delaunay meshes. Delaunay triangulations have many good properties, but the requirement that all tetrahedra be Delaunay often forces mesh generators to overrefine where boundary polygons meet at small angles---that is, they produce too many tetrahedra, making them too small. Relaxing the Delaunay property makes it possible both to reduce overrefinement and to obtain higher-quality tetrahedra.
We describe a provably good algorithm that generates high-quality meshes that are *constrained* Delaunay triangulations, rather than purely Delaunay. This change has two big advantages: it allows us to generate higher-quality tetrahedra than purely Delaunay algorithms do, and it allows us to cope much more successfully with domains that have small angles. Both theory and an implementation show that our algorithm does not overrefine near small domain angles.

Veranstalter
WIAS Berlin
Mittwoch, 14.11.2018, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Hannes Meinlschmidt, RICAM, Linz, Österreich:
Maximal regularity in optimal control for quasilinear parabolic evolution equations
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

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Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
Mittwoch, 14.11.2018, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Jürgen Pilz, Universität Klagenfurt:
The interplay between random field models for Bayesian spatial prediction and the design of computer experiments
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
In the first part of my talk, I will give an overview of recent work with my colleagues G. Spoeck and H. Kazianka in the area of Bayesian spatial prediction and design [1]-[5]. The Bayesian approach not only offers more flexibility in modeling but also allows us to deal with uncertain distribution parameters, and it leads to more realistic estimates for the predicted variances. Moreover, I will demonstrate how to apply copula methodology to Bayesian spatial modeling and use it to derive predictive distributions. I will also report on recent results for determining objective priors for the crucial nugget and range parameters of the widely used Matern-family of covariance functions. Briefly, I will also consider the problem of choosing an öptimal" spatial design, i.e. finding an optimal spatial configuration of the observation sites minimizing the total mean squared error of prediction over an area of interest. Our results will be illustrated by modeling environmental phenomena and designing a hyrogeological monitoring network in Upper Austria. In the second part of my talk I will report on modifying and transferring spatial random field models to make them accessible to the analysis of complex computer code. Over the last three decades, the design of computer experiments has rapidly developed as a statistical discipline at the intersection of the well established theories of DoE, stochastic processes, stochastic simulation and statistical parameter estimation, with the aim of approximating complex computer models to reproduce the behaviour of engineering, physical, biological, environmental and social science processes. We will focus on the use of Gaussian Processes (GPs) for the approximation of computer models, thereby stepping from simple parametric setups to using GPs as basis functions of additive models. Then we discuss the numerical problems associated with the estimation of the model parameters, in particular, the second-order (variance and correlation) parameters. To overcome these problems I will highlight joint recent work with my colleague N. Vollert [6] using Bayesian regularization, based on objective (reference) priors for the parameters. Finally, we will consider design problems associated with the search for numerical robustness of the estimation procedures. We illustrate our findings by modeling the magnetic field of a magnetic linear position detection system as used in the automotive industry. References: [1] H. Kazianka and J. Pilz: Bayesian spatial modeling and interpolation using copulas. Computers & Geosciences 37(3): 310-319, 2011 [2] H. Kazianka and J. Pilz: Objective Bayesian analysis of spatial data taking account of nugget andrange parameters. The Canadian Journal of Statistics 40(2): 304-327, 2012 [3] J. Pilz, H. Kazianka and G. Spoeck: Some advances in Bayesian spatial prediction and sampling design. Spatial Statistics 1: 65-81, 2012 [4] G. Spoeck and J. Pilz: Spatial sampling design based on spectral approximations of the errorprocess. In: Spatio-temporal design: Advances in Efficient Data Acquisition (W.G. Mueller and J. Mateu, Eds.), Wiley, New York 2013, 72-102 [5] G. Spoeck and J. Pilz: Simplifying objective functions and avoiding stochastic search algorithms in spatial sampling design. Front. Environ. Sci. 3:39: 1-22, 2015 [6] N. Vollert, M. Ortner and J. Pilz: Robust Additive Gaussian Process Models Using ReferencePriors and Cut-Off-Designs. To appear in: Applied Mathematical Modelling (2019), https://doi.org/10.1016/j.apm.2018.07.050

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Dienstag, 13.11.2018, 15:00 Uhr (WIAS-HVP-3.13)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
PhD Eric Joseph Hall, Rheinisch-Westfälische Technische Hochschule Aachen:
Causality and Bayesian network PDE for multiscale representations of porous media
mehr ... Veranstaltungsort
Weierstraß-Institut, Hausvogteiplatz 11A, 10117 Berlin, 3. Etage, Raum: 3.13

Abstrakt
Hierarchical porous media that feature properties and processes at multiple scales arise in many engineering applications including the design of novel materials for energy storage devices. Microscopic (pore-scale) properties of the media impact their macroscopic (continuum- or Darcy-scale) counterparts and understanding the relationships between processes on these two scales is essential for informing engineering decision tasks. However, microscopic properties typically exhibit complex statistical correlations that present challenges for the estimation of macroscopic quantities of interest (QoIs), e.g., global sensitivity analysis (GSA) of macroscopic QoIs with respect to microscopic material properties that respect structural constraints. We present a systematic framework for building correlations into stochastic multiscale models through Bayesian networks. This allows us to construct the joint probability density function (PDF) of model parameters through causal relationships that emulate engineering processes related to the design of hierarchical nanoporous materials. Such PDFs also serve as input for the forward propagation of parametric uncertainty; our findings indicate that the inclusion of causal relationships impacts predictions of macroscopic QoIs. To assess the impact of correlations and causal relationships between microscopic parameters on macroscopic material properties, we use a moment-independent GSA based on the differential mutual information. Our GSA accounts for the correlated inputs and complex non-Gaussian QoIs. The global sensitivity indices are used to rank the effect of uncertainty in microscopic parameters on macroscopic QoIs, to quantify the impact of causality on the multiscale model's predictions, and to provide physical interpretations of these results for hierarchical nanoporous materials.

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Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics

Veranstalter
WIAS Berlin
Dienstag, 13.11.2018, 14:15 Uhr (TUB-MA 415)
Oberseminar Nonlinear Dynamics
Dr. Hannes Stuke, Freie Universität Berlin:
A dynamical systems approach to outlier robust machine learning
mehr ... Veranstaltungsort
Technische Universität, Straße des 17. Juni 136, 10623 Berlin, MA 415

Abstrakt
We consider a typical problem of machine learning - the reconstruction of probability distributions of observed data. We introduce the so-called gradient conjugate prior (GCP) update and study the induced dynamical system. We will explain the dynamics of the parameters and show how one can use insights from the dynamical behavior to recover the ground truth distribution in a way that is more robust agains outliers. The developed approach also carries over to neutral networks.

Veranstalter
Freie Universität Berlin
WIAS Berlin
Dienstag, 13.11.2018, 13:30 Uhr (WIAS-406)
Seminar Materialmodellierung
Alex Christoph Goeßmann, Fritz-Haber-Institut der Max-Planck-Gesellschaft:
Representing crystals for kernel-based learning of their properties
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
Accurate modeling of many-body systems like crystals requires to capture their quantum-mechanical nature at the atomic scale. The solution of the associated electronic structure problem is however illusional due to the number of variables, but we obtain certain properties by computational-demanding methods like density-functional theory. In this talk, I will discuss the potential of kernel-based machine learning to circumvent this computational bottleneck and predict crystal properties. A crucial preliminary step is the representation of crystals, which has to satisfy different conditions for the learning to perform optimally.

Veranstalter
WIAS Berlin
Montag, 12.11.2018, 13:00 Uhr (WIAS-ESH)
Seminar Quantitative Biomedizin
Prof. Anna Marciniak-Czochra, Universität Heidelberg:
Post-Turing tissue pattern formation: Insights from mathematical modelling
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Cells and tissue are objects of the physical world, and therefore they obey the laws of physics and chemistry, notwithstanding the molecular complexity of biological systems. What are the mathematical principles that are at play in generating such complex entities from simple laws? Understanding the role of mechanical and mechano-chemical interactions in cell processes, tissue development, regeneration and disease has become a rapidly expanding research field in the life sciences.To reveal the patterning potential of mechano-chemical interactions, we have developed two classes of mathematical models coupling dynamics of diffusing molecular signals with a model of tissue deformation. First we derived a model based on energy minimisation that leads to 4-th order partial differential equations of evolution of infinitely thin deforming tissue (pseudo-3D model) coupled with a surface reaction-diffusion equation. The second approach (full-3D model) consists of a continuous model of large tissue deformation coupled with a discrete description of spatial distribution of cells to account for active deformation of single cells. The models account for a range of mechano-chemical feedbacks, such as signalling-dependent strain, stress, or tissue compression. Numerical simulations show ability of the proposed mechanisms to generate development of various spatio-temporal structures. We compare the resulting patterns of tissue invagination and evagination to those encountered in developmental biology. We discuss analytical and numerical challenges of the proposed models and compare them to the classical Turing patterns as well as reaction-diffusion ODE models coupling diffusion-based cell-to-cell communication with intracellular signalling.

Veranstalter
WIAS Berlin
Mittwoch, 07.11.2018, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Thomas Frenzel, WIAS Berlin:
Membrane limit for porous medium equation in weighted Wasserstein space
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
We discuss the limit passage of a porous medium equation where a membrane is enclosed by two bulk-parts. The diffusion coefficient on the membrane-part scales like its thickness. By using a gradient structure with a weighted Wasserstein dissipation we perform the analysis by means of evolutionary Gamma-convergence. The major issue is then in deriving the effective dissipation potential. As a byproduct we obtain the transmission condition that couples the equations of the remaining bulk parts on the membrane with zero thickness.

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Mittwoch, 07.11.2018, 11:30 Uhr (WIAS-406)
Seminar Interacting Random Systems
András Tóbiás, Technische Universität Berlin:
Signal to interference ratio percolation for Cox point processes
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
We study signal-to-interference plus noise ratio (SINR) percolation for Cox point processes, i.e., Poisson point processes with a random intensity measure. SINR percolation was first studied by O. Dousse et al. in the case of a two-dimensional Poisson point process. It is a version of continuum percolation where the connection between two points depends on the locations of all points of the point process. Continuum percolation for Cox point processes was recently studied by C. Hirsch, B. Jahnel and E. Cali. We study the SINR graph model for a stationary Cox point process in two or higher dimensions. We show that under suitable moment or boundedness conditions on the path-loss function and the intensity measure, this graph has an infinite connected component if the spatial density of points is large enough and the interferences are sufficiently reduced (without vanishing). This holds in all dimensions larger than 1 if the intensity measure is asymptotically essentially connected, and in two dimensions also if the intensity measure is only stabilizing but the connection radius is large. A prominent example of the intensity measure is the two-dimensional Poisson--Voronoi tessellation. We show that its total edge length in a given square has some exponential moments. We conclude that its SINR graph has an infinite cluster if the path-loss function is bounded and has a power-law decay of exponent at least 3.

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Forschungsgruppenseminar Interacting Random System

Veranstalter
WIAS Berlin
Dienstag, 06.11.2018, 15:15 Uhr (FU Berlin)
Oberseminar Nonlinear Dynamics
Prof. em. Dr. Lutz Schimansky-Geier, Humboldt-University Berlin:
Celestial mechanics of fruit flies or a theory for mushroomers
mehr ... Veranstaltungsort
Freie Universität Berlin, Arnimallee 3, 14195 Berlin, Hinterhaus, Raum 130

Abstrakt
The topic of global search in complex environments have been often investigated. But a search can also be local in the sense that it is centered at a given home position. In the latter case, the searcher does not only look for a new target but is also required to regularly return to the home position. Such behavior is typical for many insects and achieves technical importance for self-navigating robotic systems. We propose a stochastic nonlinear model for local search which does not distinguish between the two aims. The dynamics bases on an unique pursuit and escape behavior of the heading from the position vector realizing thereby optimal exploration of space and the return to the home. We discuss the mechanics of the searcher and inspect the role of noise. Such randomness is present in the decision making rule of selecting the new heading direction. We consider Levy noises with different degree of discontinuity and report about steady spatial densities for the searchers. Also we report about an optimal noise intensity that a searcher finds a target at nearby places. For this noise value the required time for finding the target becomes minimal which appears to be the consequence of different relaxation processes in the spatial and the angular dynamics. Further extensions of the model are discussed during the lecture.

Veranstalter
Freie Universität Berlin
WIAS Berlin
Dienstag, 06.11.2018, 15:15 Uhr (FU Berlin)
Oberseminar Nonlinear Dynamics
Ivan Ovsyannikov, University of Hamburg:
Birth of discrete Lorenz attractors in global bifurcations
mehr ... Veranstaltungsort
Freie Universität Berlin, Arnimallee 3, 14195 Berlin, Hinterhaus, Raum 130

Veranstalter
Freie Universität Berlin
WIAS Berlin
Dienstag, 06.11.2018, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Prof. Raul Tempone, Rheinisch-Westfälische Technische Hochschule Aachen:
Multilevel and Multi-index Monte Carlo methods for the McKean-Vlasov equation
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
In this talk, I will talk about our recent work where we address the approximation of functionals depending on a system of parti cles, described by stochastic differential equations (SDEs), in the mean-f ield limit when the number of particles is infinite. This problem is equiv alent to estimating the weak solution of the limiting McKean-Vlasov SDE. To that e nd, our approach uses systems with finite numbers of particles and a time-ste pping scheme. In this setting, there are two discretization parameters: the number of time steps and the number of particles. Based on these two parameters, we consider different variants of the Monte Carlo, Multilevel Monte Carlo (MLMC) and Multi-Index Monte Carlo methods and show that, based on som e assumptions that are verified numerically, we are able to achi eve a near- optimal work complexity in a typical setting.

Veranstalter
WIAS Berlin
Dienstag, 06.11.2018, 14:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Prof. John Meddocks, École Polytechnique Fédérale de Lausanne, Switzerland:
Estimating structured precision matrices
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
In my group's work on estimating parameter sets in coarse-grain (or multi-scale) sequence-dependent models of DNA we came across the following basic problem: For a given (positive-definite) covariance matrix (in our case estimated in standard ways from Molecular dynamics simulation time series) what is the "best fit" precision, or inverse covariance, matrix under the contraint that the precision matrix must have a prescribed block banded sparsity pattern. I will describe various known, but apparently not well known, results in this and related directions, involving maximum entropy and relative entropy and likelihood principles.

Veranstalter
WIAS Berlin
Donnerstag, 25.10.2018, 11:30 Uhr (WIAS-406)
Seminar Interacting Random Systems
Andrea Agazzi, Duke University, USA:
Large deviations and recurrence for chemical reaction networks
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
The microscopic dynamics of well-stirred networks of chemical reactions are modeled as jump Markov processes. At large volume, one may expect in this framework to have a straightforward application of large deviation theory. This is not at all true, for the jump rates are typically neither globally Lipschitz, nor bounded away from zero, with both blowup and absorption as quite possible scenarios. In joint work with Amir Dembo and Jean-Pierre Eckmann, we utilize Lyapunov stability theory to bypass this challenges and to characterize a large class of network topologies that satisfy the full Wentzell-Freidlin theory of asymptotic rates of exit from domains of attraction. The extension of such results to the estimation of transitions times between metastable states further requires the positive recurrence of such processes. This property depends critically on dynamics at of the jump Process when the concentration of some of the components are small. Precise statements on the connection between this property and the structure of the underlying network are an active area of research.

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Seminar Interacting Random Systems

Veranstalter
WIAS Berlin
Mittwoch, 24.10.2018, 15:00 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Dan Tiba, Romanian Academy, Bucharest:
Implicit parametrizations and applications in optimization and control
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
The talk will discuss necessary conditions (with less Lagrange multipliers), perturbations and general algorithms in non-convex optimization problems. Optimal control problems with mixed constraints, governed by ordinary differential equations, will also be studied in this context. The treatment is based on a recent approach to implicit systems, constructing parametrizations of the corresponding manifold, via iterated Hamiltonian equations.

Veranstalter
WIAS Berlin
Mittwoch, 24.10.2018, 14:00 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Dr. Pierluigi Colli, Università di Pavia, Italien:
Shape memory alloys, phase-field models, Cahn--Hilliard systems
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
The title collects some special keywords for research fields in the framework of phase transitions and free boundary problems describing an important research area receiving attention, interest, and a number of significant contributions from Jürgen Sprekels. This talk will offer the speaker the possibility of reviewing some models, problems, and results in the analysis and optimal control of the related systems of partial differential equations. And, more than that, it will be a pleasure for him to recall some thirty years enjoying math by his collaboration with J. Sprekels.

Veranstalter
WIAS Berlin
Mittwoch, 24.10.2018, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Dr. Evgeny Stepanov, Russian Academy of Sciences, Russische Föderation:
Hybrid control in a model from molecular biology: Between continuous and discrete
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
The central argument of the talk will be the rather modern concept in automated control: Hybrid controls through finite state machines in continuous dynamical models. It will be shown that hybrid systems present a highly complicated dynamics. Then a class of largely open problems arizing from molecular biology will be presented and studied, where the hybrid system comes out from a extra- ordinarily simple and completely classical models based on ODEs without any discrete object incorporated.

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Forschungsseminar “Mathematische Statistik”

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Dienstag, 23.10.2018, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Alexey Naumov, National Research University Higher School of Economics, Russische Föderation:
Gaussian approximations for maxima of large number of quadratic forms of high-dimensional random vectors
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

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Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics

Veranstalter
WIAS Berlin
Dienstag, 23.10.2018, 10:15 Uhr (WIAS-406)
Seminar Nichtlineare Optimierung und Inverse Probleme
Dr. Dan Tiba, Romanian Academy, Bucharest:
Optimal control methods in shape optimization
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Veranstalter
WIAS Berlin
Mittwoch, 17.10.2018, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Artur Stephan, WIAS Berlin:
On reaction systems with different time scales
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

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Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
Mittwoch, 17.10.2018, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Mark Podolskij, Aarhus University, Denmark:
Statistical inference for fractional models
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
In recent years, fractional and moving average type models have gained popularity in economics and finance. Most popular examples include fractional Brownian/stable motion, rough volatility models and Hawkes processes. In this talk we will review some existing estimation methods and present new theoretical results.

Veranstalter
WIAS Berlin
Universität Potsdam
Humboldt-Universität zu Berlin
Dienstag, 16.10.2018, 15:15 Uhr (FU Berlin)
Oberseminar Nonlinear Dynamics
Jan Sieber, University of Exeter, UK:
A nonsmooth saddle-node in a non-autonomous dynamical system
mehr ... Veranstaltungsort
Freie Universität Berlin, Arnimallee 3, 14195 Berlin, Hinterhaus, Raum 130

Veranstalter
Freie Universität Berlin
WIAS Berlin
Dienstag, 16.10.2018, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Alexander Gasnikov, WIAS Berlin:
Unified view on accelerated methods for structural convex optimization problems
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Veranstalter
WIAS Berlin
Dienstag, 16.10.2018, 13:30 Uhr (WIAS-406)
Seminar Materialmodellierung
Dr. Arik Yochelis, Ben-Gurion University of the Negev, Israel:
From solvent free to dilute electrolytes: A unified continuum approach
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Veranstalter
WIAS Berlin
Dienstag, 16.10.2018, 10:15 Uhr (WIAS-406)
Seminar Materialmodellierung
Dr. Charlotte Kuhn/Dr. Alexander Schlüter, Technische Universität Kaiserslautern:
Phase field modelling of fracture -- From a mechanics point of view
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

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Seminar Materialmodellierung

Veranstalter
WIAS Berlin
8. – 10. Oktober 2018 (WIAS-ESH)
Workshop/Konferenz: AMaSiS 2018: Applied Mathematics and Simulation for Semiconductors
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
University of Massachusetts
Dienstag, 25.09.2018, 15:00 Uhr (WIAS-HVP-3.13)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Franz Besold, Humboldt-Universität zu Berlin:
Adaptive clustering using kernel density estimators
mehr ... Veranstaltungsort
Weierstraß-Institut, Hausvogteiplatz 11A, 10117 Berlin, 3. Etage, Raum: 3.13

Abstrakt
We investigate statistical properties of a clustering algorithm suggested by Ingo Steinwart, Bharath K. Sriperumbudur and Philipp Thomann that receives level set estimates from a kernel density estimator and then estimates the first split in the density level cluster tree if such a split is present or detects the absence of such a split. Key aspects of the analysis include finite sample guarantees, consistency, rates of convergence, and an adaptive data-driven strategy for choosing the kernel bandwidth. For the rates and the adaptivity we do not need continuity assumptions on the density such as Hölder continuity, but only require intuitive geometric assumptions of non-parametric nature.

Veranstalter
WIAS Berlin
24. – 28. September 2018 (WIAS-ESH)
Workshop/Konferenz: Analysis of Evolutionary and Complex Systems
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Universität Stuttgart
University of Warwick
Dienstag, 18.09.2018, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Prof. M. Thamban Nair, Indian Institute of Technology Madras, Indien:
An inverse problem in parabolic PDEs
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

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Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics

Veranstalter
WIAS Berlin
Dienstag, 18.09.2018, 10:15 Uhr (WIAS-406)
Seminar Nichtlineare Optimierung und Inverse Probleme
Prof. Alexander Kruger, Federation University Ballarat, Australia:
Extremal principle revisited
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Veranstalter
WIAS Berlin
Donnerstag, 13.09.2018, 15:00 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Dr. Mine Akbas, Duzce University, Türkei:
The analogue of grad-div stabilization in DG methods for incompressible flows: limiting behavior and extension to tensor-product meshes
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Grad-div stabilization is a classical remedy in conforming mixed finite element methods for incompressible flow problems, for mitigating velocity errors that are sometimes called poor mass conservation. Such errors arise due to the relaxation of the divergence constraint in classical mixed methods and are excited whenever the spacial discretization has to deal with comparably large and complicated pressures. In this contribution, an analogue of grad-div stabilization for Discontinuous Galerkin methods is studied. Here, the key is the penalization of the jumps of the normal velocities over facets of the triangulation, which controls the measure-valued part of the distributional divergence of the discrete velocity solution. Our contribution is twofold: first, we characterize the limit for arbitrarily large penalization parameters, which shows that the stabilized nonconforming Discontinuous Galerkin methods remain robust and accurate in this limit; second, we extend these ideas to the case of non-simplicial meshes; here, broken grad-div stabilization must be used in addition to the normal velocity jump penalization, in order to get the desired pressure robustness effect. Several numerical examples illustrate the theory and show their relevance for the simulation of practical, nontrivial flows.

Veranstalter
WIAS Berlin
Donnerstag, 13.09.2018, 14:00 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Vojtěch Miloš, Charles University Prague:
Thermodynamics and electrochemical impedance spectroscopy - 1D model of YSZ electrolyte and triple phase boundary interface
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Electrochemical impedance spectroscopy (EIS) together with reliable models provides a powerful tool for analysing processes taking place in solid oxide cells (SOC). The presented model is derived within the framework of non-equilibrium thermodynamics [1, 2]. It is a suitable tool for description of the fundamental processes in charged mixtures [3, 4, 5], especially on interfaces, such as surface adsorption, bulk and surface diffusion, electrochemical reactions and formation of charged double layers. Special attention is dedicated to the thermodynamically consistent derivation of the boundary conditions and to handling of the electrical double layer. The contribution contains a derivation of 1D model for YSZ electrolyte and tripple boundary interface and also includes numerical solution of model partial differential equations. The implementation of finite element method (FEM) is done using open source library FEniCS. In order to explore the parametric space, EIS was simulated for a wide range of kinetic parameters of the model.
References:
1. De Groot, S. R., Mazur, P. Non-equilibrium thermodynamics., 1984.
2. Guhlke, Clemens. “Theorie der elektrochemischen Grenzfläche”, TU Berlin (2015).
3. Dreyer, W., Guhlke, C., Landstorfer, M. Electrochem. Commun. 43 (2014): 75-78.
4. Dreyer, W., Clemens, Guhlke, C., Müller, R. Ph. Chem. Chem. Ph. 17.40 (2015): 27176-27194.
5. Landstorfer, M., Guhlke, C., Dreyer, W. Electrochimica Acta 201 (2016): 187-219.


Veranstalter
WIAS Berlin
Dienstag, 11.09.2018, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Franz Besold, Humboldt-Universität zu Berlin:
Adaptive clustering using kernel density estimators
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
We investigate statistical properties of a clustering algorithm suggested by Ingo Steinwart, Bharath K. Sriperumbudur and Philipp Thomann that receives level set estimates from a kernel density estimator and then estimates the first split in the density level cluster tree if such a split is present or detects the absence of such a split. Key aspects of the analysis include finite sample guarantees, consistency, rates of convergence, and an adaptive data-driven strategy for choosing the kernel bandwidth. For the rates and the adaptivity we do not need continuity assumptions on the density such as Hölder continuity, but only require intuitive geometric assumptions of non-parametric nature.

Veranstalter
WIAS Berlin
Montag, 03.09.2018, 14:00 Uhr (WIAS-ESH)
INSTITUTSKOLLOQUIUM
Prof. Dr. Bernd Witzigmann, University of Kassel, Germany:
Carrier Injection Efficiency and Impurity Modeling in III-nitride LEDs (joint work with H. Fülle and F. Römer)
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
III-nitride based multiple quantum well structures are the active region of choice for LEDs and lasers from the ultraviolet to the visible spectral regime. For maximum efficiency, the electron and hole carrier densities need to be distributed uniformly in the quantum wells at all relevant operating conditions. We analyze the carrier injection and distribution in polar and semi-polar LEDs using a microscopic carrier transport model. It is based on drift-diffusion currents, and quantum wells are treated as scattering centers, with self-consistent kp-band structure calculation for the quantum well carriers. Tunneling currents at hetero interfaces are included via an effective potential model. Impurities such as Si, Mg and O are activated according to a Schottky-type model. Their distribution is found from experimental SIMS data. The activation mode includes both the impurity density as well as the local electric field (Poole-Frenkel effect). Comparison to experimental data reveals the following findings: the hole injection efficiency depends critically on the proximity of the Mg dopants to the p-side quantum well. At the same time, electron leakage is suppressed by ionized Mg atoms forming an electrostatic barrier, which is reduced with rising current due to saturation of the acceptor levels. Impurities that form shallow donors (e.g. oxygen) also hamper the hole injection efficiency as they reduce the Mg ionization rate. In UV LEDs, special care has to be taken for electron injection into the deep quantum wells. Therefore a high performance LED requires nanoscale control of intentional and unintentional impurity distribution. Finally, it is shown that the ideality factor of the pin-diode contains information on the injection homogeneity of the LED.

Veranstalter
WIAS Berlin
Donnerstag, 23.08.2018, 16:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Modelle der Photonik
Markus Kantner, WIAS Berlin:
Hybrid quantum-classical modeling of quantum dot based single-photon sources
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Donnerstag, 26.07.2018, 15:00 Uhr (WIAS-406)
FG Stochastische Systeme mit Wechselwirkung
Prof. Alexander Drewitz, Universität zu Köln:
Phase transitions in some percolation models with long-range correlations on general graphs
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
We consider two fundamental percolation models with long-range correlations on reasonably general and well-behaved transient graphs: The Gaussian free field and (the vacant set) of Random Interlacements. Both models have been the subject of intensive research during the last years and decades, on $Z^d$ as well as on some more general graphs. We consider their percolation phase transition and investigate a couple of interesting properties of their critical parameters, in particular the existence of a phase transition. This talk is based on joint works with A. Prevost (Koeln) and P.-F. Rodriguez (Los Angeles).

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FG 5 Seminar

Veranstalter
WIAS Berlin
Mittwoch, 18.07.2018, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Dr. Jan van Waaij, Humboldt Universität zu Berlin:
Adaptive nonparametric Bayesian methods for SDEs
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
We consider continuous time observations (Xt_0le tle T of the SDE dXt = Theta(Xt)dt + dWt: Our goal is estimating the unknown drift function Theta. Due to their numerical advantages, Bayesian methods are often used for this. In this talk I discuss optimal rates of convergence for these methods. I will start with an introduction to Bayesian methods for SDEs and what sufficient con- ditions are for posterior convergence. I show that the sufficient conditions are satisfied for the Gaussian process prior, which leads to optimal convergence rates, provided the smoothness of Theta matches the smootness of the Gaussian process. Adaptivity can be obtained by equipping the hyperparameter(s) of the Gaussian process prior with an additional prior. We discuss several choices for such hyperparameters. A new promising approach to obtain adaptivity is empirical Bayes. Here the optimal hyperparameters for the Gaussian process prior are first esti- mated from the data and then the prior with those hyperparameters plugged- in is used for the inference. This talk is based on van Waaij, 2018 and joint work with Frank van der Meulen (TU Delft), Moritz Schauer (Leiden University) and Harry van Zanten (University of Amsterdam).

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Donnerstag, 12.07.2018, 10:00 Uhr (WIAS-ESH)
Seminar Laserdynamik
Prof. Dmitry Turaev, Imperial College London, GB:
On positive metric entropy conjecture
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Mittwoch, 11.07.2018, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Martin Heida, WIAS Berlin:
Dynamic and hysteretic boundary conditions for the Mullins-Sekerka equation
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

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Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
Mittwoch, 11.07.2018, 11:00 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Dr. Ernesto Meneses Rioseco, Leibniz-Institut für Angewandte Geophysik, Hannover:
Modeling, simulation and optimization of geothermal reservoir -- THMC (thermo-hydraulical, mechanical, chemical) processes involved
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Geothermal energy constitutes an important renewable energy that can significantly contribute to the energy transition in Germany. The optimization of the geothermal energy extraction in the underground is a multi-variable and multi-scale optimization problem that has not been extensively addressed in the scientific community. Understanding and minimizing the risks, in particular induced seismicity, that the exploitation of deep geothermal energy may pose is of major public and scientific concern.
Robust mathematical models and numerical methods are required to capture the complex interplay of thermal, hydraulic, mechanical and chemical (THMC) processes in heterogeneous and anisotropic medium domains. In addition, the distribution and implementation of material properties in topologically complex 3D modeling domains is an ongoing effort. Considerable effort is required to realistically implement the complex architecture that results from highly heterogeneous and anisotropic geologic media.
This work presents a general overview of the mathematical formulations of the transport phenomena of mass, energy and momentum in porous and fractured geothermal reservoirs and emphasizes the current challenges faced for the simulation of the main risks (e.g. induced seismicity) that result from the complex interaction of THMC processes during the operation of geothermal plants and the need of complex optimization methods.
This presentation aims at encouraging the collaboration between the Leibniz Institute of Applied Geophysics (LIAG) and the Weierstrass Institute for Applied Analysis and Stochastics (WIAS) within the framework of the Leibniz MMS network.

Veranstalter
WIAS Berlin
Montag, 09.07.2018, 14:00 Uhr (WIAS-ESH)
Oberseminar Nonlinear Dynamics
Nils Berglund, Iniversite d'Orleans, Universite de Tours, CNRS, Frankreich:
Metastable dynamics of Allen--Cahn SPDEs on the torus
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Stochastic processes subject to weak noise often show a metastable behaviour, meaning that they converge to equilibrium extremely slowly; typically, the convergence time is exponentially large in the inverse of the variance of the noise (Arrhenius law). In the case of finite-dimensional Ito stochastic differential equations, the large-deviation theory developed in the 1970s by Freidlin and Wentzell allows to prove such Arrhenius laws and compute their exponent. Sharper asymptotics for relaxation times, including the prefactor of the exponential term (Eyring--Kramers laws) are known, for instance, if the stochastic differential equation involves a gradient drift term and homogeneous noise. One approach that has been very successful in proving Eyring--Kramers laws, developed by Bovier, Eckhoff, Gayrard and Klein around 2005, relies on potential theory. I will describe Eyring--Kramers laws for some parabolic stochastic PDEs such as the Allen--Cahn equation on the torus. In dimension 1, an Arrhenius law was obtained in the 1980s by Faris and Jona-Lasinio, using a large-deviation principle. The potential-theoretic approach allows us to compute the prefactor, which turns out to involve a Fredholm determinant. In dimensions 2 and 3, the equation needs to be renormalized, which turns the Fredholm determinant into a Carleman--Fredholm determinant. Based on joint work with Barbara Gentz (Bielefeld), and with Giacomo Di Gesù (Vienna) and Hendrik Weber (Warwick).

Veranstalter
Freie Universität Berlin
WIAS Berlin
Donnerstag, 05.07.2018, 16:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Modelle der Photonik
Nina Owschimikow, TU Berlin:
Carrier dynamics in InAs submonolayer quantum dots
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Mittwoch, 04.07.2018, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Jan Mankau, Technische Universität Dresden:
A descent step algorithm based on generalized gradients on sets
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
here

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Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
Dienstag, 03.07.2018, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Paolo Pigato, WIAS Berlin:
Asymptotic analysis of rough volatility models
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Veranstalter
WIAS Berlin
Freitag, 29.06.2018, 14:30 Uhr (WIAS-ESH)
Halbleiterseminar
Dr. Miguel Caro, Aalto University, Finnland:
Growth of amorphous carbon simulated with a machine-learning based interatomic potential
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

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WIAS-Halbleiterseminar

Veranstalter
WIAS Berlin
Freitag, 29.06.2018, 14:00 Uhr (WIAS-ESH)
Halbleiterseminar
Prof. Dr. Morten Willatzen, Technical University of Denmark, Dänemark:
Strain and symmetry effects in Bi2Se3 thin films using the k·p method
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

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WIAS-Halbleiterseminar

Veranstalter
WIAS Berlin
Mittwoch, 27.06.2018, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Dr. Leonid Berlyand, Pennsylvania State University, USA:
Free boundary PDE models of active gels
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
here

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
Mittwoch, 27.06.2018, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Stanislav Nagy, Charles University, Prague:
Statistical depth for functional data
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
The concept of data depth is an attempt to generalize quantiles to complex data (random vectors, random functions, or distributions on manifolds and graphs). The main idea is, for a general space S, to assign to any points from S its depth D(s; P) with respect to a probability distribution P on S. The depth D(s; P) says how "centrally located" s is with respect to P. The point maximizing D(:; P) over the sample space is a generalized median; loci of points with high depth constitute inner depth-quantile regions of P. We study depth designed for functional data, and its general framework. We show that most known functional depths can be classi ed into few groups, within which they share similar theoretical properties. We focus on uniform sample consistency results, and demonstrate that some well-known approaches to depth assessment are hardly theoretically adequate.

Veranstalter
Universität Potsdam
Humboldt-Universität zu Berlin
WIAS Berlin
Dienstag, 26.06.2018, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Alexandra Suvorikova, WIAS Berlin:
CLT in 2-Wasserstein space
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Veranstalter
WIAS Berlin
Dienstag, 26.06.2018, 13:30 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Prof. Traian Iliescu, Virginia Tech, USA:
Physically-constrained data-driven correction for reduced order modeling of fluid flows
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
We propose a data-driven correction reduced order model (DDC-ROM) framework for the numerical simulation of fluid flows, which can be formally written as DDC-ROM = Galerkin-ROM + Correction. The new DDC-ROM is constructed by using ROM spatial filtering and data-driven ROM closure modeling (for the Correction term). Furthermore, we propose a physically-constrained DDC-ROM (CDDC-ROM), which aims at improving the physical accuracy of the DDC-ROM. The new physical constraints require that the CDDC-ROM operators satisfy the same type of physical laws (i.e., the Correction term's linear component should be dissipative and the Correction term's nonlinear component should conserve energy) as those satisfied by the fluid flow equations. To implement these physical constraints, in the data-driven modeling step, we replace the unconstrained least squares problem with a constrained least squares problem. We perform a numerical investigation of the new CDDC-ROM and standard DDC-ROM for a 2D channel flow past a circular cylinder at Reynolds numbers Re=100, Re=500, and Re=1000. To this end, we consider a reproductive regime as well as a predictive (i.e., cross-validation) regime in which we use as little as 50 % of the original training data. The numerical investigation clearly shows that the new CDDC-ROM is significantly more accurate than the DDC-ROM in both regimes.

Veranstalter
WIAS Berlin
25. – 27. Juni 2018 (TU-H2036)
Workshop/Konferenz: Interplay of Random Media and Stochastic Interface Models (RMSI2018)
mehr ... Veranstaltungsort
Technische Universität, Straße des 17. Juni 135, 10623 Berlin, Main building, Room H 2036

Veranstalter
Technische Universität Berlin
Mittwoch, 20.06.2018, 14:30 Uhr (WIAS-ESH)
INSTITUTSKOLLOQUIUM
Prof. Dr. Michael C. Ferris, Computer Sciences Department and Wisconsin Institute for Discovery, University of Wisconsin-Madison, USA:
Modelling 100 percent renewable electricity
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
In the past few decades, power grids across the world have become dependent on markets that aim to efficiently match supply with demand at all times via a variety of pricing and auction mechanisms. These markets are based on models that capture interactions between producers, transmission and consumers. Energy producers typically maximize profits by optimally allocating and scheduling resources over time. A dynamic equilibrium aims to determine prices and dispatches that can be transmitted over the electricity grid to satisfy evolving consumer requirements for energy at different locations and times. Computation allows large scale practical implementations of socially optimal models to be solved as part of the market operation, and regulations can be imposed that aim to ensure competitive behaviour of market participants. The recent explosion in the use of renewable supply such as wind, solar and hydro has led to increased volatility in this system. We develop models that aim to ensure enough generation capacity for the long term under various constraints related to environmental concerns, and consider the recovery of costs for this enhanced infrastructure. We demonstrate how risk can impose significant costs on the system that are not modeled in the context of socially optimal power system markets and highlight the use of contracts to reduce or recover these costs. We also outline how battery storage can be used as an effective hedging instrument.

Veranstalter
WIAS Berlin
Mittwoch, 20.06.2018, 10:00 Uhr (WIAS-406)
Forschungsseminar Mathematische Statistik
Prof. Zuoqiang Shi, Tsinghua University, Beijing, China:
PDE-based methods for Interpolation on High Dimensional Point Cloud
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
Interpolation on high dimensional point cloud provides a fundamental model in many data analysis and machine learning problems. In this talk, we will present some PDE based methods to do interpolation on point cloud. Applications in image processing and data analysis are shown to demonstrate the performance of our methods.

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Dienstag, 19.06.2018, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Prof. Zuoqiang Shi, Tsinghua University, Beijing, China:
Low dimensional manifold model for image processing
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
In this talk, I will introduce a novel low dimensional manifold model for image processing problem. This model is based on the observation that for many natural images, the patch manifold usually has low dimension structure. Then, we use the dimension of the patch manifold as a regularization to recover the original image. Using some formula in differential geometry, this problem is reduced to solve Laplace-Beltrami equation on manifold. The Laplace-Beltrami equation is solved by the point integral method. Numerical tests show that this method gives very good results in image inpainting, denoising and super-resolution problem. This is joint work with Stanley Osher and Wei Zhu.

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Dienstag, 19.06.2018, 10:15 Uhr (WIAS-406)
Seminar Nichtlineare Optimierung und Inverse Probleme
Prof. Dr. Guanghui Hu, Beijing Computational Science Research Center, China:
Outgoing radiation conditions for the Helmholtz equation in inhomogeneous periodic media
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Veranstalter
WIAS Berlin
Montag, 18.06.2018, 15:00 Uhr (WIAS-406)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Dr. Johannes Pfefferer, TU München:
hp-finite elements for fractional diffusion
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
In this talk we introduce and analyze a numerical scheme based on hp-finite elements to solve boundary value problems involving the spectral fractional Laplacian. The approach is based on a reformulation of the problem posed on a semi-infinite cylinder in one more spatial dimension. After a suitable truncation of this cylinder, the resulting problem is discretized with linear finite elements in the original domain and with hp-finite elements in the extended direction. The proposed approach yields a reduction of the computational complexity in terms of degrees of freedom and even has slightly improved convergence properties compared to the state-of-the-art discretization using linear finite elements for both the original domain and the extended direction. The performance of the method is illustrated by numerical experiments.

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Joint Research Seminar on Mathematical Optimization / Non-smooth Variational Problems and Operator Equations

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
18. – 20. Juni 2018 (WIAS-ESH)
Workshop/Konferenz: Nonlinear Dynamics in Semiconductor Lasers
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Donnerstag, 14.06.2018, 15:00 Uhr (WIAS-405-406)
Seminar Numerische Mathematik
Prof. Liyong Shen, University of the Chinese Academy of Sciences, Beijing:
Implicitizing rational tensor product surfaces using the resultant of three moving planes
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Abstrakt
Implicitizing rational surfaces is a fundamental computational task in Computer Graphics and Computer Aided Design. Ray tracing, collision detection, and solid modeling all benefit from implicitization procedures for rational surfaces. Recently a popular method is based on analysis of moving planes (syzygies): construct the implicit matrix using moving planes or compute the resultant of μ-basis. The moving planes method relies on the construction of appropriate moving planes. The μ-bases for rational surfaces are difficult to compute. Moreover, μ-bases for a rational surface often have high degrees, so these resultants generally contain many extraneous factors.
Here we develop fast algorithms to implicitize rational tensor product surfaces by computing the resultant of three moving planes corresponding to three syzygies with low degrees. These syzygies are easy to compute, and the resultants of the corresponding moving planes generally contain fewer extraneous factors than the resultants of the moving planes corresponding to μ-bases. We predict and compute all the possible extraneous factors that may appear in these resultants.

Veranstalter
WIAS Berlin
Donnerstag, 14.06.2018, 14:00 Uhr (WIAS-405-406)
Seminar Numerische Mathematik
Dr. Myfanwy Evans, TU Berlin:
Surfaces and tangling
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Abstrakt
In biological systems, hyperbolic surfaces are readily identifiable as critical to function. This talk will introduce a few interesting systems that motivate thinking in this direction, followed by a more formal mathematical process that aims to probe the realm of complicated three-dimensional structure, in particular highly tangled materials.

Veranstalter
WIAS Berlin
14. – 16. Juni 2018 (WIAS-ESH)
Workshop/Konferenz: 9th Annual ERC Berlin-Oxford Young Researchers Meeting on Applied Stochastic Analysis
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Technische Universität Berlin
University of Oxford
Mittwoch, 13.06.2018, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Markus Mittnenzweig, WIAS Berlin:
Hydrodynamic limit and large deviations for reaction--diffusion master equations
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

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Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
Mittwoch, 13.06.2018, 13:00 Uhr (WIAS-ESH)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Dr. Kathrin Welker, Universität Trier:
Constrained shape optimization problems in shape spaces
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Shape optimization problems arise frequently in technological processes which are modelled in the form of partial differential equations (PDEs) or variational inequalities (VIs). In many practical circumstances, the shape under investigation is parametrized by finitely many parameters, which on the one hand allows the application of standard optimization approaches, but on the other hand limits the space of reachable shapes unnecessarily. In this talk, the theory of shape optimization is connected to the differential-geometric structure of shape spaces. In particular, efficient algorithms in terms of shape spaces and the resulting framework from infinite dimensional Riemannian geometry are presented. In this context, the space of H^1/2-shapes is defined. The H^1/2-shapes are a generalization of smooth shapes and arise naturally in shape optimization problems. Moreover, VI constrained shape optimization problems are treated from an analytical and numerical point of view in order to formulate approaches aiming at semi-smooth Newton methods on shape vector bundles. Shape optimization problems constrained by VIs are very challenging because of the necessity to operate in inherently non-linear and non-convex shape spaces. In classical VIs, there is no explicit dependence on the domain, which adds an unavoidable source of non-linearity and non-convexity due to the non-linear and non-convex nature of shape spaces.

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Joint Research Seminar on Mathematical Optimization / Non-smooth Variational Problems and Operator Equations

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Mittwoch, 13.06.2018, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Alain Celisse, Universite des Sciences et Technologies de Lille, France:
Early stopping rule and discrepancy principle in reproducing kernel Hilbert spaces
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
The main focus of this work is on the nonparametric estimation of a regression function by means of reproducing kernels and several iterative learning algorithms such as gradient descent, spectral cut-off, Tikhonov regularization,... First, we exploit the general framework of filter estimators to provide a unified analysis of these different algorithms. With the Tikhonov regularization, we will discuss the in uence of the parametrization on the interaction between the condition number of the Gram matrix and the number of iterations. More generally, we also discuss existing links between the qualification assumption and the used filter estimators. Second, we introduce an early stopping rule derived from the so-called discrepancy principle. Its behavior is compared with that one of other existing stopping rules and analyzed through the understanding of the dependence of the empirical risk with respect to in uential parameters (Gram matrix eigenvalues, cumulative step size, initialization). An oracle-type inequality is derived to quantify the finite-sample performance of the proposed stopping rule. The practical performance of the procedure is also empirically assessed from several simulation experiments.

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Dienstag, 12.06.2018, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Pavel Dvurechensky, WIAS Berlin:
Distributed calculation of Wasserstein barycenters
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
We study the semi-discrete optimal transport problem of decentralized distributed computation of a discrete approximation for regularized Wasserstein barycenter of a finite set of continuous probability measures distributedly stored over a network. Particularly, we assume that there is a network of agents/machines/computers where each agent holds a private continuous probability measure, and seek to compute the barycenter of all the measures in the network by getting samples from its local measure and exchanging information with its neighbors. Motivated by this problem, we develop a novel accelerated primal-dual stochastic gradient descent method for general stochastic convex optimization problems with linear equality constraints, and modify it for decentralized distributed optimization setting to generate a new algorithm for the distributed semi-discrete regularized Wasserstein barycenter problem. The proposed algorithm can be executed over arbitrary networks that are undirected, connected and static, using the local information only. Moreover, we show explicit non-asymptotic convergence rates in terms of the problem parameters. Finally, we show the effectiveness of our method on the distributed computation of the regularized Wasserstein barycenter of univariate Gaussian and von Mises distributions; as well as some applications to image aggregation.

Veranstalter
WIAS Berlin
Dienstag, 12.06.2018, 10:15 Uhr (WIAS-406)
Seminar Nichtlineare Optimierung und Inverse Probleme
Dr. Michael Peters, Eidgenössische Technische Hochschule Zürich, Schweiz:
Uncertainty quantification for elliptic PDEs
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Veranstalter
WIAS Berlin
Montag, 11.06.2018, 16:00 Uhr (WIAS-406)
Seminar Laserdynamik
Iva Bačić, University of Belgrade:
Stochastic dynamics in systems of adaptively coupled active rotators
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Veranstalter
WIAS Berlin
Montag, 11.06.2018, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Dr. Dorothee Knees, Universität Kassel:
Rate-independent processes with time-discontinuous data
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Rate-independent processes typically exhibit nonsmooth behavior even if the given data are smooth in time. If the underlying energy functional is nonconvex, then discontinuous solutions appear. Meanwhile, there is a wide variety of solution concepts dealing with such discontinuities. However, in these investigations it is typically assumed that the given data (applied loads in the mechanical context) are in some sense differentiable with respect to time. But this excludes processes, where for instance the loads are switched on or off instantaneously. The aim of this lecture is to discuss for a prototypical rate-independent model with a nonconvex energy functional a solution concept that allows for discontinuous data. The idea is to start from a vanishing viscosity procedure and to identify as precisely as possible the limit process. If time permits the results are put into relation with the results from [1], where the convex case is treated in the framework of Kurzweil-integrals.
The lecture relies on joint work with Chiara Zanini (Politecnico di Torino) and discussions with Pavel Krejci.
[1] P. Krejci, M. Liero, Rate independent Kurzweil processes, Appl. Math., Praha, 54(2):117--145,2009.

Weitere Informationen
Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
Mittwoch, 06.06.2018, 16:00 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Seungchan Ko, Oxford University, GB:
Analysis and approximation of incompressible chemically reacting non-Newtonian fluids
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
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Weitere Informationen
Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
Mittwoch, 06.06.2018, 10:00 Uhr (WIAS-HVP-3.13)
Forschungsseminar Mathematische Statistik
Igor Cialenco, Illinois Institute of Technology:
Parameter estimation problems for parabolic SPDEs
mehr ... Veranstaltungsort
Weierstraß-Institut, Hausvogteiplatz 11A, 10117 Berlin, 3. Etage, Raum: 3.13

Abstrakt
In the rst part of the talk we will discuss the parameter estimation problem using Bayesian approach for the drift coecient of some linear (parabolic) SPDEs driven by a multiplicative noise of special structure. We assume that one path of the rst N Fourier modes of the solution are continuously observed over a nite time interval, and we derive Bayesian type estimators for the drift coefficient. As custom for Bayesian statistics, we prove a Bernstein--Von Mises theorem for the posterior density, and consequently, we derive some asymptotic properties of the proposed estimators, as N goes to in nity. In the second part of the talk we will study parameter estimation problems for discretely sampled SPDEs. We will discuss some general results on derivation of consistent and asymptotically normal estimators based on computation of the p-variations of stochastic processes and their smooth perturbations, that consequently are conveniently applied to SPDEs. Both the drift and the volatility cofficients are estimated using two sampling schemes - observing the solution at a xed time and on a discrete spatial grid, and at a xed space point and at discrete time instances of a nite interval. The theoretical results will be illustrated via numerical examples.

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Dienstag, 05.06.2018, 15:15 Uhr (WIAS-ESH)
Oberseminar Nonlinear Dynamics
Prof. Dr. Jens Starke, Universität Rostock:
Multiscale analysis of collective behaviour in particle systems
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
Freie Universität Berlin
WIAS Berlin
Dienstag, 05.06.2018, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Priv.-Doz. Dr. Peter Mathé, WIAS Berlin:
Nyström subsampling for regression functions of low smoothness
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Veranstalter
WIAS Berlin
Dienstag, 05.06.2018, 10:15 Uhr (WIAS-406)
Seminar Nichtlineare Optimierung und Inverse Probleme
Daniel Walter, Technische Universität München:
Sparse sensor placement for inverse problems: From finite to infinite dimensional parameters
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Veranstalter
WIAS Berlin
Mittwoch, 30.05.2018, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. José Francisco Rodrigues, University of Lisbon, Portugal:
Evolutionary quasi-variational and variational inequalities with constraints on the derivatives
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
In this talk I shall present a general framework for the study of the existence of quasi-variational and variational solutions to a class of nonlinear evolution systems in convex sets of Banach spaces describing constraints on a linear combination of partial derivatives of the solutions, which is a joint work with Lisa Santos and Fernando Miranda. The quasi-linear operators are of monotone type, but are not required to be coercive for the existence of weak solutions, which is obtained by a double penalisation/regularisation for the approximation of the solutions. In the case of time-dependent convex sets that are independent of the solution, we show also the uniqueness and the continuous dependence of the strong solutions of the variational inequalities, extending previous results to a more general framework and including several models that will be surveyed.

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Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
Mittwoch, 30.05.2018, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Florian Schäfer, California Institute of Technology, USA:
Compression, inversion, and approximate PCA of dense kernel matrices at near-linear computational complexity
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Many popular methods in machine learning, statistics, and uncertainty quantification rely on priors given by smooth Gaussian processes, like those obtained from the Matern covariance functions. Furthermore, many physical systems are described in terms of elliptic partial differential equations. Therefore, implicitely or explicitely, numerical simulation of these systems requires an ecient numerical representation of the corresponding Green's operator. The resulting kernel matrices are typically dense, leading to (often prohibitive) O (N^2) or O(N^3) computational complexity. In this work, we prove rigorously that the dense N x N kernel matrices obtained from elliptic boundary value problems and measurement points distributed approximately uniformly in a d-dimensional domain can be Cholesky factorised to accuracy  in computational complexity O(N log^2(N) log^2d(N/e)) in time and O (N log(N) log d(N/e)) in space. For the closely related Matern covariances we observe very good results in practise, even for parameters corresponding to non-integer order equations. As a byproduct, we obtain a sparse PCA with near-optimal low-rank approximation property and a fast solver for elliptic PDE. We emphasise that our algorithm requires no analytic expression for the covariance function. Our work is inspired by the probabilistic interpretation of the Cholesky factorisation, the screening effect in spatial statistics, and recent results in numerical homogenisation.

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Forschungsseminar “Mathematische Statistik”

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Dienstag, 29.05.2018, 15:15 Uhr (FU Berlin)
Oberseminar Nonlinear Dynamics
Murad Banaji, Middlesex University London, England:
Inheritance of behaviours in bio/chemical networks
mehr ... Veranstaltungsort
Freie Universität Berlin, Arnimallee 3, 14195 Berlin, Hinterhaus, Raum 130

Abstrakt
Consider the following imprecise questions about dynamical systems with some network structure: If a small network is embedded in a larger one, then must the larger one "inherit" some behaviours of the smaller one? Can the presence of certain "motifs" be sensibly used to make predictions about dynamical behaviours of a network? Once such questions are phrased precisely, the answers turn out to depend on the class of networks studied, the dynamical behaviour of interest and, crucially, the chosen notion of ßubnetwork". My talk will focus on some recent results on chemical reaction networks (CRNs), where the behaviours in question are multistationarity and oscillation. Network modifications such as adding or deleting reactions, adding or deleting species from reactions, and inserting intermediates into a reaction are considered for their effects on multistationarity and oscillation. For example, it can be shown that both behaviours are inherited in the induced subnetwork partial ordering on fully open CRNs, but the same does not hold for general CRNs. Under certain mild assumptions, growing a CRN by inserting intermediates into reactions preserves the capacity for multistationarity in a CRN. The theorems are proved using essentially local techniques -- the implicit function theorem and regular and singular perturbation theory -- and hold for various kinetics including (but not exclusively) mass action kinetics. The results on multisationarity are joint with Casian Pantea.

Veranstalter
Freie Universität Berlin
WIAS Berlin
Dienstag, 29.05.2018, 13:30 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Prof. Grigory Panasenko, University of Lyon, Frankreich & University of Chile:
Asymptotic analysis of wave propagation in a laminated beam with contrasting stiffness of the layers
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
The wave equation in a thin laminated beam is considered in the case when the ratio of the thickness and the length is a small parameter e, while the ratio of the stiffness coefficients of the layers is a great parameter w. The question of an approximation of such a beam by the one dimensional models is discussed. It is shown that the classical homogenized approximation is valid for the case of sufficiently small value of the product of e square by w. If this product is not small then the classical model should be replaced by a multicomponent homogenization model analogous to [1, 2]. The waves in such multicomponent beam model can have multiple wave velocities. This result generalizes [3] Section 2.6.

[1] Panasenko, G.P. “Averaging of processes in strongly in homogeneous media”, Doklady Akademii Nauk SSSR, 1988, 298, 1, 76--79 (in Russian). English transl. in Soviet Phys. Dokl., 1988, 33.
[2] Panasenko, G.P. “Multicomponent homogenization of processes in strongly non-homogeneous structures” Mathematics USSRSbornik, 1990, 181, 1, 134--142 (in Russian); English transl. in Math. USSR Sbornik, 1991, 69, 1, 143--153.
[3] Panasenko, G.P. “Multi-Scale Modelling for Structures and Composites”, Springer, Dordrecht, 2005, 398 pp.

Veranstalter
WIAS Berlin
Montag, 28.05.2018, 14:00 Uhr (WIAS-ESH)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Dr. Francisco José Romero Hinrichsen, ETH Zurich, Schweiz:
Dynamical super-resolution with applications to ultrafast ultrasound
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Recently there has been a successful development in ultrasound imaging, increasing significantly the sampling rate and therefore enhancing this imaging's capacities. In particular, for vessel imaging, the use of microbubble tracking allows us to super-resolve blood vessels, and by estimating the particles' speeds inside them, it is possible to calculate the vessels' diameters. In this context, we model the microbubble tracking problem, formulating it in terms of a sparse spike recovery problem in the phase space (the position and velocity space), that allows us to obtain simultaneously the speed of the microbubbles and their location. This leads to an L1 minimization algorithm for point source tracking, that promises to be faster than current alternatives.

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Joint Research Seminar on Mathematical Optimization / Non-smooth Variational Problems and Operator Equations

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Mittwoch, 23.05.2018, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Dr. Pierluigi Colli, Università di Pavia, Italien:
A Cahn--Hilliard system with convection and dynamic boundary conditions: Well-posedness and optimal velocity control
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Weitere Informationen
Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
Mittwoch, 23.05.2018, 13:00 Uhr (WIAS-ESH)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Dr. Amal Alphonse, WIAS:
Directional differentiability for elliptic QVIs of obstacle type
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Quasi-variational inequalities (QVIs) are generalisations of variational inequalities where the associated constraint set is no longer explicitly given but it depends on the solution itself. In this talk, we present some work on the directional differentiability of the multi-valued mapping that takes the source term of a QVI onto the set of solutions. This result represents a first step in the study of differential sensitivity of QVIs in infinite dimensions. We also discuss an application to thermoforming and show some numerical experiments.

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Joint Research Seminar on Mathematical Optimization / Non-smooth Variational Problems and Operator Equations

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Mittwoch, 23.05.2018, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Markus Reiß/Randolf Altmeyer, Humboldt-Universität zu Berlin:
A nonparametric estimation problem for linear SPDEs
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
(Please find here)

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Dienstag, 22.05.2018, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Tobias Kley, Humboldt-Universität zu Berlin:
Sequential detection of structural changes in irregularly observed data
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
Online surveillance of time series is traditionally done with the aim to identify changes in the marginal distribution under the assumption that the data between change-points is stationary and that new data is observed at constant frequency. In many situations of interest to data analysts, the classical approach is too restrictive to be used unmodified. We propose a unified system for the monitoring of structural changes in streams of data where we use generalised likelihood ratio-type statistics in the sequential testing problem, obtaining the flexibility to account for the various types of changes that are practically relevant (such as, for example, changes in the trend of the mean). The method is applicable to sequences where new observations are allowed to arrive irregularly. Early identification of changes in the trend of financial data can assist to make trading more profitably. In an empirical illustration we apply the procedure to intra-day prices of components of the NASDAQ-100 stock market index. This project is joint work with Piotr Fryzlewicz.

Veranstalter
WIAS Berlin
Dienstag, 22.05.2018, 10:15 Uhr (WIAS-406)
Seminar Nichtlineare Optimierung und Inverse Probleme
Prof. Dr. Masahiro Yamamoto, University of Tokyo, Japan:
Inverse problems of determining moving sources in diffusion equations
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Veranstalter
WIAS Berlin
Donnerstag, 17.05.2018, 16:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Modelle der Photonik
Prof. Ulf Leonhardt, Weizmann Institute of Science, Israel:
Light at the horizon
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Donnerstag, 17.05.2018, 15:00 Uhr (WIAS-406)
FG Stochastische Systeme mit Wechselwirkung
Prof. Dr. Nicolas Petrelis, Université de Nantes, Frankreich:
Scaling limit of the Interacting partially directed self-avoiding walk at criticality
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

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Seminar FG 5

Veranstalter
WIAS Berlin
Donnerstag, 17.05.2018, 10:15 Uhr (WIAS-406)
Seminar Nichtlineare Optimierung und Inverse Probleme
Caroline Geiersbach, Universität Wien, Österreich:
A projected stochastic gradient algorithm for optimization with random elliptic PDE constraints
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Veranstalter
WIAS Berlin
Mittwoch, 16.05.2018, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Alexander Mielke, WIAS Berlin:
Finding limiting dissipative potentials via EDP convergence
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
A gradient structure for an evolutionary system is a triple, a so-called gradient system, consisting of a state space, a possibly time-dependent energy functional, and a dissipation potential that induces the given evolutionary system. The gradient structure, which is not unique, contains additional thermodynamical information, e.g. on the fluctuations in an associated microscopic model. Considering a family of gradient systems depending on a small parameter, it is natural to ask for the limiting or effective gradient system if the parameter tends to 0. We propose some ideas towards the derivation of the effective gradient structure that are based on De Giorgi's Energy-Dissipation Principle (EDP). We discuss several versions of EDP convergence and show by examples that the theory is flexible enough to allow situations where starting from quadratic dissipation potentials we arrive at effective dissipation potentials that are no longer quadratic.

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Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
Mittwoch, 16.05.2018, 13:00 Uhr (WIAS-ESH)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Ass.-Prof. Dr. Lothar Banz, Universität Salzburg, Österreich:
A posteriori error estimates for $hp$-dual mixed finite elements by implicit reconstruction
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
A posteriori error estimates are derived for dual mixed methods of $hp$-adaptive finite elements for variational equations as well as for variational inequalities. The error control relies on the use of a special, but never computed, $H^1$-reconstruction of the non-smooth discrete potential. Thus, no post-processing reconstruction (and therewith no additional computation) is needed. Moreover, the use of the discrete potential instead of its reconstruction improves significantly the error estimation in terms of the numerical efficiency indices which are nearly constant and close to one in the numerical experiments. Numerical experiments demonstrate the convergence rates and the efficiency indices of these a posteriori error estimates in $h$- and $hp$-adaptivity.

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Joint Research Seminar on Mathematical Optimization / Non-smooth Variational Problems and Operator Equations

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Mittwoch, 16.05.2018, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Moritz Jirak, TU Braunschweig; Martin Wahl, Humboldt-Universität zu Berlin:
Relative perturbation bounds with applications to empirical covariance operators
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
A problem of fundamental importance in quantitative science is to estimate how a perturbation of a covariance operator effects the corresponding eigenvalues and eigenvectors. Due to its importance, this problem has been heavily investigated and discussed in the literature. In this talk, we present general perturbation expansions for a class of symmetric, compact operators. Applied to empirical covariance operators, these expansions allow us to describe how perturbations carry over to eigenvalues and vectors in terms of necessary and sucient conditions, characterising the perturbation transition. We demonstrate the usefulness of these expansions by discussing pca and fpca in various setups, including more exotic cases where the data is assumed to have high persistence in the dependence structure or exhibits (very) heavy tails. This talk is jointly given by Moritz Jirak and Martin Wahl, and divided into two parts

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Dienstag, 15.05.2018, 15:15 Uhr (WIAS-ESH)
Oberseminar Nonlinear Dynamics
Dr. Alexander Linke, WIAS Berlin:
On the role of the Helmholtz--Leray projector for a novel pressure-robust discretization theory for the incompressible Navier--Stokes equations
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
The talk discusses several physical regimes of the incompressible Navier--Stokes equations with respect to the role of the pressure and the role of the Helmholtz--Leray projector in the Navier--Stokes momentum balance. It is emphasized that not the forces in the momentum balance themselves matter for the complexity of the flow field, but their Helmholtz--Leray projector. Since the Helmholtz--Leray projector vanishes for arbitrary gradient fields, a semi norm and a corresponding equivalence class of forces play naturally a major role for the evolution of incompressible flows. Novel pressure-robust mixed finite element methods are designed for an appropriate discrete treatment of this equivalence class of forces. On the contrary, classical, (only) inf-sup stable mixed methods do not care about the existence of such an equivalence class. In order to deliver accurate simulation results, they have to resort to expensive high order ansatz spaces. Finally, the talk will indicate suitable applications for efficient and accurate low-order pressure-robust mixed methods like flows with electrolytes and flows with stagnation points, and it will discuss possible extensions to compressible low-Mach number flows.

Veranstalter
Freie Universität Berlin
WIAS Berlin
Dienstag, 15.05.2018, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Michele Coghi, WIAS Berlin:
Interacting particle systems
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Veranstalter
WIAS Berlin
Mittwoch, 09.05.2018, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Ansgar Jüngel, Technische Universität Wien, Österreich:
Cross-diffusion systems with entropy structure
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
here

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Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
Mittwoch, 09.05.2018, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Gitta Kutyniok, Technische Universität Berlin:
Optimal approximation with sparsely connected deep neural networks
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Despite the outstanding success of deep neural networks in real-world applications, most of the related research is empirically driven and a mathematical foundation is almost completely missing. One central task of a neural network is to approximate a function, which for instance encodes a classi cation task. In this talk, we will be concerned with the question, how well a function can be approximated by a neural network with sparse connectivity. Using methods from approximation theory and applied harmonic analysis, we will derive a fundamental lower bound on the sparsity of a neural network. By explicitly constructing neural networks based on certain representation systems, so-called -shearlets, we will then demonstrate that this lower bound can in fact be attained. Finally, we present numerical experiments, which surprisingly show that already the standard backpropagation algorithm generates deep neural networks obeying those optimal approximation rates. This is joint work with H. B?olcskei (ETH Zurich), P. Grohs (Uni Vienna), and P. Petersen (TU Berlin).

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Dienstag, 08.05.2018, 15:15 Uhr (WIAS-ESH)
Oberseminar Nonlinear Dynamics
Prof. Marc Timme, Technische Universität Dresden:
Network dynamics as an inverse problem
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Freie Universität Berlin
Dienstag, 08.05.2018, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Valeriy Avanesov, WIAS Berlin:
SDE's, Gaussian processes, clusters, etc.
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Veranstalter
WIAS Berlin
Dienstag, 08.05.2018, 13:30 Uhr (WIAS-406)
Seminar Materialmodellierung
Dr. Simon Praetorius, Technische Universität Dresden:
From individual motion to collective cell migration
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
here

Veranstalter
WIAS Berlin
Montag, 07.05.2018, 13:15 Uhr (WIAS-406)
Halbleiterseminar
Dr. Annegret Glitzky, WIAS Berlin:
Thermodynamically consistent models for coupling reaction-diffusion effects in bulk and interfaces
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

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Halbleiterseminar

Veranstalter
WIAS Berlin
Donnerstag, 03.05.2018, 15:00 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Lukas Feierabend, Universität Duisburg-Essen:
Model development for flowing slurry electrodes in zinc-air batteries
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Slurry or suspension electrodes are gaining a renewed interest for large-scale energy storage technologies due to potentially higher energy densities compared to redox flow batteries [1]. A suspension electrode typically consists of a liquid electrolyte and a solid material, which is chemically active and electrically conducting. For the investigated zinc-air flow batteries employing a suspension electrode, microscopic zinc particles are suspended in an aqueous potassium hydroxide electrolyte [2, 3]. Sedimentation of the metallic zinc is minimized by adding a gelling agent to the electrolyte. The gelling agent and the high particle loading lead to a pseudo-plastic rheological behavior. When the flowable suspension electrode is pumped through the channels of the zinc-air fuel cell setup, the dynamic percolation network within the suspension dictates the active electrode surface area and the maximum discharge power density. Therefore, it is desirable to investigate the influence of the local flow conditions on the particle percolation and consequently the electrochemical performance of the battery cell with adequate simulation methods. In the developed three-dimensional numerical model for the flowing suspension electrode, the complex, non-Newtonian two-phase flow is approximated by a coupling of an Eulerian continuum description for the electrolyte and a discrete, Lagrangian particle description for the motion and interaction of the microscopic particles. The partial differential equations for momentum, species, charge and energy are discretized by the finite volume method and implemented in the OpenFOAM library [4]. The particle motion including multiple simultaneous particle contacts is described with the discrete element method using the LIGGGHTS library [5]. Coupling between the particle and fluid phase is realized with the CFD-DEM method using the CFDEM library [5], where an empirical description accounts for the momentum exchange between the non-Newtonian fluid and the densely distributed particle assemblies. Due to relatively high particle concentrations, the volume displacement by the particles is considered in the electrolyte continuum model. A half-cell model for the anode part of the zinc-air flow battery is implemented, which accounts for the flow characteristics via the described CFD-DEM coupling method. Simultaneously, the charge and species transport is considered according to the classic porous electrode theory by Newman [6] adapted with newer formulations for concentrated electrolytes [7]. In contrast to electrodes with a static porous matrix, the heterogeneous local porosities change temporally depending on the evolving particle distributions. Additionally, the active electrode surface area in each finite volume is dependent on the percolation network from the considered local point to the current collector surface. To estimate the basic parameters for the charge transfer and species transport, a flat-plate electrode with a flowing electrolyte without particles is investigated. In this rather simple reference case, discontinuities of the current density close to the electrode-electrolyte-interface could be observed, which should be inherently excluded due to the model formulations. These inconsistencies will be discussed in detail. Funding by the Federal Ministry for Economic Affairs and Energy (BMWi) of the project â?œZnPLUS -Wiederaufladbare Zink-Luft-Batterien zur Energiespeicherungâ?? and the project â?œZnMobil - Mechanisch und elektrisch wiederaufladbare Zink-Luft-Batterie für automobile Anwendungenâ?? is gratefully acknowledged.

Veranstalter
WIAS Berlin
Donnerstag, 03.05.2018, 14:00 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Dr. Holger Stephan, WIAS Berlin:
One million perrin pseudo primes including a few giants
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Pseudoprimes are integers that are no primes but behave like them in some sense. Suppose we have a theorem like the following: If n is a prime, then statement A(n) holds. In general, the opposite is not true: It may be that A(n) holds, but n is a composite number, a so-called pseudo prime with respect to statement A. Pseudoprimes are interesting if they are very rare, as for instance Perrin's pseudoprimes, the smallest of which is 271441. The talk introduces pseudoprimes which are based on recurrent sequences. In addition, some new numerical results on Perrin's pseudoprimes and a fast algorithm for their calculation are presented.

Veranstalter
WIAS Berlin
Mittwoch, 25.04.2018, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Nicolai Baldin, University of Cambridge, GB:
Optimal link prediction with matrix logistic regression
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
In this talk, we will consider the problem of link prediction, based on partial observation of a large network, and on side information associated to its vertices. The generative model is formulated as a matrix logistic regression. The performance of the model is analysed in a high-dimensional regime under a structural assumption. The minimax rate for the Frobenius-norm risk is established and a combinatorial estimator based on the penalised maximum likelihood approach is shown to achieve it. Furthermore, it is shown that this rate cannot be attained by any (randomised) algorithm computable in polynomial time under a computational complexity assumption. (joint work with Q. Berthet)

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Forschungsseminar “Mathematische Statistik”

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Montag, 23.04.2018, 14:00 Uhr (WIAS-ESH)
Seminar Quantitative Biomedizin
Prof. Sarah Waters, University of Oxford, GB:
Fluid dynamical models for tissue engineering
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Tissue engineers aim to grow tissues in vitro to replace those in the body that have been damaged through age, trauma or disease. A common approach is to seed cells within a scaffold which is then cultured in a bioreactor. The key challenge it to provide the appropriate mechanical and biochemical cellular environment that promotes tissue growth in vitro. Fluid flows have a key role to play in addressing this challenge, as they can provide mechanical cues to cells (e.g. via fluid shear and pressure), and enhance the delivery of nutrients and growth factors (via advection). In this talk, I will explore how mechanistic mathematical models, in combination with state-of-the art experimental studies, can provide quantitative insights into the interplay between the fluid flows and the resulting tissue growth. Quantitative understanding of this interplay offers the exciting potential to manipulate the experimental design (e.g. scaffold porosity, bioreactor operating conditions) to establish defined mechanical cues that enhance the generation of complex 3D tissues in vitro.

Veranstalter
WIAS Berlin
Mittwoch, 18.04.2018, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Mario Varga, Technische Universität Dresden:
Stochastic unfolding and homogenization
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Weitere Informationen
Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
Mittwoch, 18.04.2018, 15:00 Uhr (WIAS-406)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Dr. Pavel Dvurechensky:
First-order optimization methods with inexact information about the objective function value and its gradient
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
In this talk I will discuss first-order methods with inexact oracle for finite-dimensional optimization. Oracle model of optimization methods assumes that, given a point, the oracle returns some information on the objective function at this point. In the case of first-order optimization methods, this information is the function value and its gradient at this point. I will start with convex problems, inexact oracle defined in the work by O. Devolder, F. Glineur, Yu. Nesterov, Math. Prog., 2014, and convergence rates for gradient descent and accelerated gradient descent in this case. I will also describe an extension for non-convex problems. Then I will discuss some ideas on how these methods potentially can be extended and applied for infinite-dimensional problems. If time allows, I will cover other optimization problems and methods, which I work with. Among others, optimal transport problem and an accelerated gradient descent for its solution, randomized optimization methods, such as random coordinate descent and random derivative-free method, variational inequalities, saddle-point problems and first-order methods for their solution.

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Joint Research Seminar on Mathematical Optimization / Non-smooth Variational Problems and Operator Equations

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Mittwoch, 18.04.2018, 13:30 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Prof. Paul Deuring, Université du Littoral “Côte d'Opale”, Frankreich:
Stabilitäts- und Fehlerschranken einer FE-FV-Diskretisierung von Konvektions-Diffusionsgleichung: exponentielle Abhängigkeit von Parametern
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Wir betrachten eine FE-FV-Methode zur Bestimmung von Näherungslösungen von Konvektions-Diffusionsgleichungen. Bei dieser Methode wird der Diffusionsterm mittels Crouzeix-Raviart-Elementen diskretisiert, während man den Konvektionsterm durch baryzentrische finite Volumen approximiert. Wir gehen der Frage nach, ob sich Stabilitäts- und Fehlerschranken für diese Methode finden lassen, die von keiner Größe exponentiell abhängen, insbesondere nicht vom Kehrwert des Diffusionskoeffizienten.

Veranstalter
WIAS Berlin
Mittwoch, 18.04.2018, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Dr. Alexandra Suvorikova, WIAS Berlin:
Gaussian process forecast with multidimensional distributional input
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
In this work, we focus on forecasting a Gaussian process indexed by probability distributions. We introduce a family of positive definite kernels constructed with the use of optimal transportation distance and provide their probabilistic understanding. The technique allows to forecast efficiently Gaussian processes, which opens new perspective in Gaussian process modelling.

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Dienstag, 17.04.2018, 15:15 Uhr (WIAS-ESH)
Oberseminar Nonlinear Dynamics
Misha Zaks, Universität Potsdam:
Collective dynamics of globally coupled excitable units
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
Freie Universität Berlin
WIAS Berlin
Dienstag, 17.04.2018, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Prof. Peter Friz, WIAS und TU Berlin:
“Algebraic statistics meets rough paths”, joint work with C. Armendola and B. Sturmfels
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
The signature of a parametric curve is a sequence of tensors whose entries are iterated integrals. This construction is central to the theory of rough paths in stochastic analysis. It is here examined through the lens of algebraic geometry. We introduce varieties of signature tensors for both deterministic and random paths. For the former, we focus on piecewise linear paths, on polynomial paths, and on varieties derived from free nilpotent Lie groups. For the latter, we focus on Brownian motion and its mixtures.

Veranstalter
WIAS Berlin
Donnerstag, 12.04.2018, 14:00 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Prof. Michael J. Neilan, University of Pittsburgh, USA:
Inf-sup stable Stokes pairs on barycentric refinements producing divergence-free approximations
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
We construct several stable finite element pairs for the Stokes problem on barycentric refinements in arbitrary dimensions and for any polynomial degree. A key feature of the spaces is that the divergence maps the discrete velocity space onto the discrete pressure space; thus, when applied to models of incompressible flows, the pairs yield divergence-free velocity approximations. The key ingredients to prove these results are local inf-sup stability estimates and a modification of Bernardi-Raugel bubble functions. This is joint work with Johnny Guzman.

Veranstalter
WIAS Berlin
Dienstag, 10.04.2018, 10:15 Uhr (WIAS-406)
Seminar Nichtlineare Optimierung und Inverse Probleme
Prof. Dr. Ozan Öktem, KTH Royal Institute of Technology, Schweden:
Recent advances in using machine learning for image reconstruction
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
The talk will outline recent approaches for using (deep) convolutional neural networks to solve a wide range of inverse problems, such as image reconstruction in medical tomography. A key element is to use a neural network architecture for reconstruction that includes physics based models that describe how data is generated as well as its statistical properties. Another is the possibility to integrate complex task related a priori information and elements of decision making into the reconstruction procedure. The resulting approach outperforms current state-of-the-art in terms of 'quality', computational speed and there is no need to manually set parameters as with variational methods. Furthermore, the amount of training data and network size can be kept surprisingly small.

Veranstalter
WIAS Berlin
Dienstag, 27.03.2018, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Elena Ivanova, AOK:
Community detection on evolving graphs
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
We consider algorithms for detecting the cluster structure in a continual evolving decentralised graph with large number of vertices. As a model for the graph we use the evolving stochastic block model, where only the set of edges changes over time. Algorithms with low upper error bounds will be presented.

Veranstalter
WIAS Berlin
Dienstag, 27.03.2018, 13:30 Uhr (WIAS-406)
Seminar Materialmodellierung
Dr. Esteban Meca, University of Cordoba, Spanien:
Localized instabilities in phase-changing systems: The effect of elasticity
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Veranstalter
WIAS Berlin
Dienstag, 20.03.2018, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Benjamin Stemper, WIAS und TU Berlin:
Advances in calibrating rough stochastic volatility models via neural networks
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
We consider rough stochastic volatility models where the driving noise of volatility has fractional scaling, in the “rough" regime of Hurst parameter H < 1/2. This regime recently attracted a lot of attention both from the statistical and option pricing point of view for its ability to better fit market data than conventional stochastic volatility models like Heston. A drawback of the new regime is that efficient pricing schemes are yet unknown, making calibration - where repetitive evaluation of the pricing functional is required - prohibitively expensive. In our last talk, we envisioned a calibration scheme using deep learning where the expensive pricing functional is learned by an artificial neural network. In this talk, we report on our progress and share some challenges encountered on the way. Work in progress, joint with Christian Bayer.

Veranstalter
WIAS Berlin
Donnerstag, 08.03.2018, 16:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Modelle der Photonik
Sabrina Pickartz:
Optical soliton control by interactions with dispersive waves
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
Mittwoch, 07.03.2018, 14:00 Uhr (WIAS-ESH)
Seminar Materialmodellierung
Dr. Matthias Liero, WIAS Berlin:
Modeling and simulation of charge transport in organic semiconductors via kinetic and drift-diffusion models
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
The use of organic materials in electronic applications such as displays, photovoltaics, lighting, or transistors, has seen an substantial increase in the last decade. This is mainly due to the lower production cost, sustainability, and flexibility. Moreover, the toolbox of organic chemistry opens an enormous potential for new device concepts. In contrast to classical semiconductors such as silicon or gallium-arsenide, charge-transport in organic materials happens via temperature activated hopping transport of electrons or holes between adjacent molecules. Here, the crucial feature is the random alignment of the molecule, which leads to a disordered system with Gaussian distributed energy levels. A common approach to simulate the transport of charge carriers in organic materials is based on a master equation description of the hopping transport and kinetic Monte-Carlo methods. However, the computational costs of this approach are typically very high and the treatment of complicated multi-dimensional device structures is very challenging. Moreover, the inclusion of multi-physics effects such as heat flow is out of scope. The latter, in particular, is of high importance as organic devices show a strong interplay between electrical current and heat flow. Here, drift-diffusion models provide an immense advantage. In this talk, we give an overview over the two modeling approaches and give an outlook to future challenges.

Veranstalter
WIAS Berlin
Dienstag, 06.03.2018, 15:15 Uhr (WIAS-ESH)
Seminar Laserdynamik
Dr. Otti D'Huys, Aston University, GB:
Dynamics of delay-coupled networks with time-varying connections
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

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Seminar Laserdynamik

Veranstalter
WIAS Berlin
Donnerstag, 01.03.2018, 14:00 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Dr. Franco Dassi, Politecnico di Milano, Italien:
Recent advancements of the Virtual Element Method in 3D
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
The Virtual Element Method is a novel way to discretize a partial differential equation. It avoids the explicit integration of shape functions and introduces an innovative construction of the stiffness matrix so that it acquires very interesting properties and advantages. One among them is the possibility to apply the VEM to general polygonal/polyhedral domain decomposition, also characterized by non-conforming and non-convex elements.
In this talk we focus on the definition/construction of the Virtual Element functional spaces in three dimensions and how apply this new strategy to set a standard Laplacian problem in 3D.
Finally we test the method to show its robustness with respect to element distortion and the polynomial approximation degree $k$. Then, we move to more involved cases: convection-diffusion-reaction problems with variable coefficients and magnetostatic Maxwell equations.

Veranstalter
WIAS Berlin
Mittwoch, 28.02.2018, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Moritz Egert, Université de Paris-Sud, Frankreich:
How half-order time derivatives help us to better understand parabolic equations
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
In the study of parabolic problems, the prototypes of which are related to the heat operator ∂ t - Δ, an old but often overlooked approach is to decompose the time-derivative into two fractional derivatives of order 1/2. Depending on ones preferences, this can either be considered 'natural' since it brings back the parabolic homogeneity, for instance when the elliptic part is interpreted in the weak sense via a sesquilinear form, or rather 'peculiar' as it introduces non-local operators in the context of local equations.

In this talk I shall touch on some recent results that used half-order time-derivatives in a crucial way, placing an emphasis on my own contributions obtained with several collaborators. This includes improvements of time-regularity for solutions to parabolic systems with measurable coefficients of second as well as fractional order, non-autonomous maximal regularity for the related initial value problem, a new approach to parabolic boundary value problems, and the solution of the parabolic Kato square root problem.

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin

Montag, 26.02.2018, 14:00 Uhr (WIAS-ESH)
Seminar Laserdynamik
Dr. Karin Mora, Universität Paderborn:
Says Newton to Hopf and Hogkin--Huxley: Vibrating! Impacting! Exciting!
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Mittwoch, 21.02.2018, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar) & Seminar Materialmodellierung
Dr. Marco Morandotti, TU München:
Dimension reduction in the context of structured deformations
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
The theory of structured deformations shows good potential to deal with mechanical problems where multiple scales and fractures are present. Math- ematically, it amounts to relaxing a given energy functional and to show also the relaxed one has an integral representation. In this seminar, I will focus on a problem for thin objects: the derivation of a 2D relaxed energy via dimension reduction from a 3D energy, incorporat- ing structured deformations in the relaxation procedure. I will discuss the two-step relaxation (first dimension reduction, then structured deformations and vice versa) and I will compare it with another result in which the two relaxation procedures are carried out simultaneously. An explicit example for purely interfacial initial energies will complete the presentation. These results have been obtained in collaboration with G. Carita, J. Matias, and D.R. Owen.

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Dienstag, 20.02.2018, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Pavel Dvurechensky, WIAS Berlin:
Faster algorithms for (regularized) optimal transport
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
We propose an alternative to ubiquitous Sinkhorn algorithm for solving regularized optimal transport problem. Our approach is based on accelerated gradient descent applied to the dual problem and allows to solve not only entropy-regularized optimal transport problem, but also squared-2-norm-regularized OT problem, which produces sparser optimal transport plan approximation. We prove that, our algorithm, allows to solve non-regularized optimal transport problem with support size $p$ up to accuracy $epsilon$ in $Oleft(minleftfracp^9/4epsilon, fracp^2epsilon^2 rightright)$ arithmetic operations, which is better than state-of-the-art result $Oleft(fracp^2epsilon^3right)$.

Veranstalter
WIAS Berlin
Donnerstag, 15.02.2018, 16:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Modelle der Photonik
Jan-Philipp Koester, Ferdinand-Braun-Institut Berlin:
Simulation of pulsed laser diodes driven far above threshold with WIAS--BALaser and comparison with experiment
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Mittwoch, 14.02.2018, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Wolfgang Dreyer, WIAS Berlin:
Modeling of non-Newtonian fluids without material frame indifference
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
In the fifties the modeling of non-Newtonian fluids initiated the search for invariant time derivatives with respect to certain space time transformations. Then Euclidean Transformations were selected to establish the Principle of Material Frame Indifference and Nth Grade Fluids. However, the two concepts lead to serious inconsistencies between thermodynamic consistent models and experimental observations. In 1986 the subject has been resolved in a remarkable paper by I. Müller and K. Wilmanski. We discuss alternative non-Newtonian fluids models of Maxwell-type and of Balance-type within Continuum Thermodynamics as it was laid down by D. Bothe and W. Dreyer.

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
Dienstag, 13.02.2018, 15:15 Uhr (FU-140)
Oberseminar Nonlinear Dynamics
Marek Fila, Comenius University Bratislava, Slovakia:
A Gagliardo--Nirenberg-type inequality and its applications to decay estimates for solutions of a degenerate parabolic equation
mehr ... Veranstaltungsort
Freie Universität Berlin, Arnimallee 7, 14195 Berlin, Hinterhaus, Raum: 140

Abstrakt
We discuss a Gagliardo-Nirenberg-type inequality for functions with fast decay. We use this inequality to derive upper bounds for the decay rates of solutions of a degenerate parabolic equation. Moreover, we show that these upper bounds, hence also the Gagliardo-Nirenberg-type inequality, are sharp in an appropriate sense. The talk will consist of two parts - an introduction of one hour and a presentation of proofs of also one hour.

Veranstalter
Freie Universität Berlin
WIAS Berlin
Dienstag, 13.02.2018, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Martin Redmann, MPI Magdeburg:
Solving parabolic rough partial differential equations using regression
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
In this talk, we discuss the numerical approximation of parabolic partial differential equations (PDEs) driven by a rough path which, e.g., can be the lift of a path of a fractional Brownian motion. By using the Feynman-Kac formula, the solution can be represented as the expected value of a functional of the corresponding hybrid Stratonovich-rough differential equation. A time-discretisation of this equation and a Monte Carlo regression in the spatial variable lead to an approximation of the solution to the rough PDE. We analyse the regression error and provide several numerical experiments to illustrate the performance of our method.

Veranstalter
WIAS Berlin
Mittwoch, 07.02.2018, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Gitta Kutyniok, Technische Universität Berlin:
Optimal approximation with sparsely connected deep neural networks
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Despite the outstanding success of deep neural networks in real-world applications, most of the related research is empirically driven and a mathematical foundation is almost completely missing. One central task of a neural network is to approximate a function, which for instance encodes a classi cation task. In this talk, we will be concerned with the question, how weural network with sparse connectivity. Using methods from approximation theory and applied harmonic analysis, we will derive a fundamental lower bound on the sparsity of a neural network. By explicitly constructing neural networks based on certain representation systems, so-called -shearlets, we will then demonstrate that this lower bound can in fact be attained. Finally, we present numerical experiments, which surprisingly show that already the standard backpropagation algorithm generates deep neural networks obeying those optimal approximation rates. This is joint work with H. B?olcskei (ETH Zurich), P. Grohs (Uni Vienna), and P. Petersen (TU Berlin).

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Dienstag, 06.02.2018, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Mario Maurelli, WIAS Berlin:
McKean-Vlasov SDEs with irregular drift: large deviations for particle approximation
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
McKean-Vlasov SDEs are SDEs where the coefficients depend on the law of the solution to the SDE. Their interest is in the links with nonlinear PDEs on one side (the SDE-related Fokker-Planck equation is nonlinear) and with interacting particles on the other side: the McKean-Vlasov SDE can be approximated by a system of weakly coupled SDEs. In this talk we consider McKean-Vlasov SDEs with irregular drift: though well-posedness for this SDE is not known, we show a large deviation principle for the corresponding interacting particle system. This implies, in particular, that any limit point of the particle system solves the McKean-Vlasov SDE. The proof combines rough paths techniques and an extended Varadhan lemma. This is a joint work with Thomas Holding.

Veranstalter
WIAS Berlin
Freitag, 02.02.2018, 14:30 Uhr (WIAS-ESH)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Thomas Ruf, Universität Augsburg:
On a variational approach to the nonlinear wave equation
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
In a 2012 paper, E. Serra and P. Tilli proved a conjecture by E. de Giorgi stating that global weak solutions of wave equations such as w” - Delta w + wlwl^(p-2) = 0 on R^+ times  R^d can be obtained as limits of minimizers of suitable variational functionals. We generalize this proof to equations of the form w” - Delta w + f_w (t; x; w) = 0 with p-growth conditions on f, also replacing R^d by an arbitrary open set O subset R^d and suitable boundary conditions.

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Joint Research Seminar on Mathematical Optimization / Non-smooth Variational Problems and Operator Equations

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
Donnerstag, 01.02.2018, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Prof. Alexander Gasnikov, Moscow Institute of Physics and Technology, Russia:
Recent developments of accelerated gradient descent, part II
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Veranstalter
WIAS Berlin
Donnerstag, 01.02.2018, 15:00 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Laura Blank, WIAS Berlin:
Towards a simple and robust finite element method for the numerical simulation of porous media flow
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
The topic of this talk is the numerical simulation of flow through porous media based on the Brinkman model as a unified framework that allows the transit between Darcy and Stokes problems. Therefore an unconditionally stable low order finite element approach, which is robust with respect to the physical parameters, is proposed. This approach is based on the combination of stabilized equal order finite elements with a non-symmetric (penalty-free) Nitsche method for the weak imposition of essential boundary conditions. Focusing on the two-dimensional case, optimal a priori error estimates in a mesh-dependent norm, which allows to extend the results also to the Stokes and Darcy limits, are obtained.

Veranstalter
WIAS Berlin
Donnerstag, 01.02.2018, 14:00 Uhr (WIAS-ESH)
Seminar Quantitative Biomedizin
Dr. Edoardo Sinibaldi, Italian Institute of Technology:
Selected modeling approaches for biomedical applications and biorobotics tools
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
The effective deployment of theranostic agents (including drugs), smart?materials?based systems and interfaces to target regions (prospectively) in the human body where to perform the sought actions also requires theoretical models and physical tools. Modeling the delivery and the (remote) actuation/stimulation of the aforementioned agents/systems/interfaces, in particular, permits to compensate for experimental conditions hard to characterize and to better interpret the experimental results, thus paving the way for quantitative therapy design and control. This is complemented by model-based design of related experimental devices, flexible tools able to safely navigate anatomical pathways, and miniaturized effectors. In this talk I will firstly present the solution to an inverse problem, namely to determine the velocity profile in a vessel cross-section starting from the flow-rate, which is relevant to pulsatile biological flows (blood and cerebro-spinal fluid flows), with application, e.g., to magnetic particle targeting. Then I will address numerical models for intrathecal drug-delivery (drug infusion in the cerebro-spinal fluid, also accounting for transport to the spinal cord) and simplified analytical models for intra-tissue drug-delivery (particle transport into porous/poroelastic media, with application to intratumoral thermotherapy). Finally, I will overview model-based approaches for: piezoelectric nanoparticle-mediated cell stimulation; real-scale physical models of the blood-brain barrier; flexible biorobotics probes; miniature (bioinspired) actuators and effectors.

Veranstalter
WIAS Berlin
Mittwoch, 31.01.2018, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Alexander Linke, WIAS Berlin:
On the role of the Helmholtz--Leray projector for a novel pressure-robust discretization theory for the incompressible Navier--Stokes equations
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
The talk discusses several physical regimes of the incompressible Navier--Stokes equations with respect to the role of the pressure and the role of the Helmholtz--Leray projector in the Navier--Stokes momentum balance. It is emphasized that not the forces in the momentum balance themselves matter for the complexity of the flow field, but their Helmholtz--Leray projector. Since the Helmholtz--Leray projector vanishes for arbitrary gradient fields, a semi norm and a corresponding equivalence class of forces play naturally a major role for the evolution of incompressible flows. Novel pressure-robust mixed finite element methods are designed for an appropriate discrete treatment of this equivalence class of forces. On the contrary, classical mixed methods do not care about the existence of such an equivalence class. In order to deliver accurate simulation results, they have to resort to expensive high order ansatz spaces. Finally, the talk will indicate suitable applications for efficient and accurate low-order pressure-robust mixed methods like flows with electrolytes, flows with stagnation points and flows with a dominant Coriolis force.

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Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
Mittwoch, 31.01.2018, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Elisabeth Gassiat, Universite Paris-Sud:
Estimation of the proportion of explained variation in high dimensions
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
The estimation of the heritability of a phenotypic trait based on genetic data may be set as the estimation of the proportion of explained variation in high dimensional linear models. I will be interested in understanding the impact of: not knowing the sparsity of the regression parameter, not knowing the variance matrix of the covariates on minimax estimation of heritability. In the situation where the variance of the design is known, I will present an estimation procedure that adapts to unknown sparsity. When the variance of the design is unknown and no prior estimator of it is available, I will show that consistent estimation of heritability is impossible. (Joint work with N. Verzelen, and PHD thesis of A. Bonnet).

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Mittwoch, 24.01.2018, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Hagen Neidhardt, WIAS Berlin:
On the spectral shift function: Recent results
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Weitere Informationen
Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
Mittwoch, 24.01.2018, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Andreas Maurer, München:
Concentration for functions of bounded interaction
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Some multivariate functions have the property that their variation in any argument does not change too much when another argument is modi ed. The talk will give some examples and concentrates on the random variable W obtained by applying such a function to a vector of independent variables. Functions with weakly interacting arguments share some important properties with sums: the expectation of W can be estimated by a version of Bernstein's inequality and its variance can be tightly estimated in terms of an iid sample, which has only one datum more than the function has arguments. There is also a version of the central limit theorem.

Veranstalter
Universität Potsdam
Humboldt-Universität zu Berlin
WIAS Berlin
Dienstag, 23.01.2018, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Paolo Pigato, WIAS Berlin:
The exploding skew of implied volatility
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
We show how a simple local volatility model, with volatility function discontinuous at-the-money, is capable of reproducing the explosion of the implied volatility skew, observed on empirical data. We use such local volatility as a leverage function, in a stochastic-local volatility model, obtaining good fits of the whole implied volatility surface on empirical observations.

Veranstalter
WIAS Berlin
Dienstag, 23.01.2018, 14:00 Uhr (WIAS-406)
Seminar Materialmodellierung
Prof. Jan Giesselmann, RWTH Aachen:
Modelling error estimates and model adaptation in compressible flows
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
tba

Veranstalter
WIAS Berlin
Freitag, 19.01.2018, 14:00 Uhr (WIAS-ESH)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Prof. Dr. Sören Bartels, Albert-Ludwigs-Universität Freiburg:
Finite element methods for nonsmooth problems and application to a problem in optimal insulation
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Nonsmooth problems arise in the mathematical modeling of contact and obstacle problems, the description of plastic material behavior, and mathematical image processing. The unknown functions are typically characterized as minimizers of nondifferentiable functionals. Numerical schemes approximately solve these problems either via duality methods or classically by making use of appropriate regularizations. In the talk we discuss the discretization and iterative solution of a model problem defined on functions of bounded variation. The numerical analysis of finite element discretizations leads to reduced convergence rates which can be improved using adaptive mesh refinement. Suitable iterative solution procedures are ADMM schemes, for which we propose an automatic step size adjustment strategy, and gradient flows, for which we demonstrate the unconditional stability of a semi-implicit time discretization. The methods are applicable in the numerical determination of optimal insulating films for heat conducting bodies. Below a critical value of available insulation mass an unexpected break of symmetry occurs.

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Joint Research Seminar on Mathematical Optimization / Non-smooth Variational Problems and Operator Equations

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
Donnerstag, 18.01.2018, 14:00 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Prof. Wolfgang Dreyer, WIAS Berlin:
Modeling of non-Newtonian fluids without material frame indifference
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
In the fifties the modeling of non-Newtonian fluids initiated the search for invariant time derivatives with respect to certain space time transformations. Then Euclidean Transformations were selected to establish the Principle of Material Frame Indifference and moreover, the introduction of Nth grade fluids. However, the two concepts lead to serious inconsistencies in both thermodynamic modeling and experimental observations. In 1986 the subject has been resolved in a remarkable paper by I. Müller and K. Wilmanski. In this lecture we show how an appropriate model of non-Newtonian fluids may be incorporated in Continuum Thermodynamics as it was laid down by D. Bothe and W. Dreyer.

Veranstalter
WIAS Berlin
Mittwoch, 17.01.2018, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Dr. Jürgen Sprekels, WIAS Berlin:
Cahn--Hilliard systems with general fractional-order operators
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Weitere Informationen
Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
Mittwoch, 17.01.2018, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Anthony Nouy, École Centrale de Nantes, Frankreich:
Learning high-dimensional functions with tree tensor networks
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Tensor methods are among the most prominent tools for the approximation of high- dimensional functions. Such approximation problems naturally arise in statistical learning, stochastic analysis and uncertainty quanti cation. In many practical situations, the approximation of high- dimensional functions is made computationally tractable by using rank-structured approximations. In this talk, we give an introduction to tree-based (hierarchical) tensor formats, which can be interpreted as deep neural networks with particular architectures. Then we present adaptive algorithms for the approximation in these formats using statistical methods.

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Dienstag, 16.01.2018, 15:15 Uhr (FU-140)
Oberseminar Nonlinear Dynamics
Jia-Yuan Dai, Freie Universität Berlin:
Existence of local solutions of Gowdy spacetimes
mehr ... Veranstaltungsort
Freie Universität Berlin, Arnimallee 7, 14195 Berlin, Hinterhaus, Raum: 140

Abstrakt
We consider a class of Gowdy spacetimes that reduces the Einstein's field equation to a system of two semilinear wave equations, by assuming a universe without matter, in which the gravitational wave fronts repeat in space and are mutually parallel. To prove the existence of local solutions of the system, we add a linear perturbation and seek periodic-in-space solutions. The idea of the proof is to apply the Lyapunov-Schmidt reduction. We will solve a related small-divisor problem and discuss how to design the correct functional setting that fits to the nonlinearity. This ongoing research is a joint work with Dr. Hannes Stuke.

Veranstalter
Freie Universität Berlin
WIAS Berlin
Dienstag, 16.01.2018, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Prof. Alexander Gasnikov, Moscow Institute of Physics and Technology, Russia:
Recent developments of accelerated gradient descent, part I
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Veranstalter
WIAS Berlin
Dienstag, 16.01.2018, 10:15 Uhr (WIAS-406)
Seminar Nichtlineare Optimierung und Inverse Probleme
Dr. Ingo Bremer, WIAS Berlin:
Introduction to a framework for nonlinear optimization with adaptation to probabilistic constraints
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Veranstalter
WIAS Berlin
Donnerstag, 11.01.2018, 16:00 Uhr (WIAS-ESH)
Seminar Laserdynamik
Prof. Andrey L. Shilnikov, Neuroscience Institute, Georgia State University, USA:
Torus bifurcations in slow fast systems
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Donnerstag, 11.01.2018, 15:00 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Dr. Amer Rasheed, Lahore University of Management Sciences, Pakistan:
Influence of magnetic field on dendrites during solidification of binary mixtures
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
A phase field model has been proposed which incorporates convection and magnetic field in an isothermal environment. A numerical scheme is proposed and numerical analysis of model in two-dimensional geometry is performed. The numerical stability and error analysis of this approximation scheme which is based on mixed finite-element method are performed. An application of a nickel-copper binary alloy is considered. Influence of various magnetic fields on the dendrites during the solidification process has been discussed.

Veranstalter
WIAS Berlin
Donnerstag, 11.01.2018, 14:00 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Prof. Gert Lube, Georg-August-Universität Göttingen:
Why exactly divergence-free H(div)-conforming FEM for transient incompressible flows?
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Recent results show that one can reconstruct classical inf-sup stable H1-conforming FEM for incompressible flow problems to be pressure-robust. Exactly divergence-free FEM are pressure-robust per construction. In particular, exactly divergence-free H(div)-conforming FEM combine excellent stability and conservation properties with minimal stabilization. We show that the numerical analysis of such methods allows to separate the (static) linear Stokes regime and the nonlinear dynamic regime. Semi-robust error estimates w.r.t. the Reynolds number will be given. In a hybridized form, H(div)-conforming FEM are suitable for large scale computation. Applications of the approach to two- and three-dimensional problems of vortex dynamics will be presented for high Reynolds numbers.

Veranstalter
WIAS Berlin
Donnerstag, 11.01.2018, 10:00 Uhr (WIAS-ESH)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Dr. Tobias Kluth, Universität Bremen:
Model-based magnetic particle imaging
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Magnetic particle imaging (MPI) is a tracer-based imaging modality developed to detect the concentration of superparamagnetic iron oxide nanoparticles. It is highly sensitive to the nanoparticle's nonlinear response to a dynamic applied magnetic field. Model-based reconstruction techniques are still not able to reach the quality of data-based approaches in which the linear system function is determined by a time-consuming measurement process. Possible reasons include the relaxation behavior of nanoparticles in fast changing magnetic fields. However, the equilibrium model described by the Langevin function is still used to predict the system behavior. In this context we discuss the ill-posedness of the imaging problem. We further focus on the model-based MPI reconstruction problem incorporating deviations in the forward operator. This is illustrated by initial results from real data using a regularized total least squares approach.

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Mittwoch, 10.01.2018, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Antoine Chambaz, Universität Paris Descartes, Frankreich:
An introduction to targeted learning
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Coined by Mark van der Laan and Dan Rubin in 2006, targeted learning is a general approach to learning from data that reconciles machine learning and statistical inference. On the one hand, machine learning" refers to the estimation of in nite-dimensional features of the law of the data, P, for instance a regression function. Machine learning algorithms are versatile, and produce (possibly highly) data-adaptive estimators. Driven by the need to make accurate predictions, they do not care so much about the assessment of prediction uncertainty. On the other hand, statistical inference" refers to the estimation of nite-dimensional parameters of P, for instance a measure of association with a causal interpretation. It focuses on the construction of con dence regions or the development of hypotheses tests. Emphasis is placed on robustness (guaranteeing that one goes to the truth even under mild and reasonable assumptions on P), eciency (trying to draw as much information from the data as possible), and controlling the asymptotic levels or type I errors.

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Donnerstag, 21.12.2017, 14:15 Uhr (WIAS-HVP-4.13)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Dr. Guozhi Dong, Humboldt-Universität zu Berlin:
Regularization methods and nonlinear PDEs for solving inverse and imaging problems
mehr ... Veranstaltungsort
Weierstraß-Institut, Hausvogteiplatz 11A, 10117 Berlin, 4. Etage, Raum: 4.13

Abstrakt
In this talk, I will present some recent developments on regularization methods in image sciences. I shall show a tiny background and also mention some of the state of the art in this area. The focus will be then to discuss some non-convex regularization models for specific problems which suffer from displacement errors. The non-convex energy functionals reveal to have tight connections to some nonlinear (geometric) partial differential equations, e.g. mean curvature flows. Finally, I will show some numerical results with some discussions on the algorithms.

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Joint Research Seminar on Mathematical Optimization / Non-smooth Variational Problems and Operator Equations

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
18. – 19. Dezember 2017 (WIAS-ESH)
Workshop/Konferenz: Dynamics of novel mode-locked and frequency-swept lasers
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Donnerstag, 14.12.2017, 14:00 Uhr (WIAS-ESH)
Seminar Materialmodellierung
Bartlomiej Matejczyk, University of Warwick, GB:
Macroscopic models for ion transport in nanoscale pores
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
During this talk, we discuss ionic transport through confined geometries. Our problem concerns modelling ionic flow through nanopores and ion channels. We present different methods of engineering the pores together with its characteristics. Next, we comment on the challenges in simulating the flow efficiently. In the second part, we present a derivation of 1D asymptotic reduction of the Poisson-Nernst-Planck model applied to long and narrow pores. We discuss numerical schema and compare the results the solution of the two-dimensional system of equations.

Veranstalter
WIAS Berlin
Mittwoch, 13.12.2017, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Joachim Rehberg, WIAS Berlin:
On maximal parabolic regularity for non-autonomous parabolic operators, Part 2
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Weitere Informationen
Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
Mittwoch, 13.12.2017, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Dr. Fabian Dunker, University of Canterbury, Christchurch, New Zealand:
Multiscale tests for shape constraints in linear random coefficient models
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
A popular way to model unobserved heterogeneity is the linear random coecient mo- del Y i = i;1Xi;1 + i;2Xi;2 + ::: + i;dXi;d. We assume that the observations (Xi; Yi); i = 1; :::; n, are i.i.d. where Xi = (Xi;1; :::;Xi;d) is a d-dimensional vector of regressors. The random coecients i = ( i;1; :::; i;d); i = 1; :::; n, are unobserved i.i.d. realizations of an unknown d-dimensional distribu- tion with density f independent of Xi. We propose and analyze a nonparametric multi-scale test for shape constraints of the random coecient density f . In particular we are interested in con dence sets for slopes and modes of the density. The test uses the connection between the model and the d-dimensional Radon transform and is based on Gaussian approximation of empirical processes. This is a joint work with K. Eckle, K. Proksch, and J. Schmidt-Hieber.

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Dienstag, 12.12.2017, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Franz Besold, Humboldt-Universität zu Berlin:
Persistence diagrams
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
In this talk we introduce persistence diagrams. These can be used as a tool to infer topological information from noisy data. To do that, we review simplicial and singular homology. Persistence diagrams have originally be defined for functions on topological spaces, but can be more generally defined using persistence modules. Stability results ensuring that close data sets have close persistence diagrams show that persistence diagrams are well-suited to deal with real-life data. Applications include data smoothing or recovering topological features of manifolds from a sampled point cloud.

Veranstalter
WIAS Berlin
Montag, 11.12.2017, 14:00 Uhr (WIAS-ESH)
INSTITUTSKOLLOQUIUM
Jürgen Borchardt/Peter Mathé, WIAS:
Der block-orientierte Prozesssimulator BOP: Was er kann und was er tat.
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
BOP dient zur Prozesssimulation, insbesondere zum Lösen großer DAE Systeme (differential-alegebraic systems). Über viele Jahre hinweg wurden mit BOP Simulationen von Gas/Öl-Turbinen vorgenommen, in der Kooperation mit Alstom, Switzerland. Um eine derartige praxisbezogene Zusammenarbeit langfristig zu gestalten, wurden die Fähigkeiten des solvers weiterentwickelt. Peripher wurden verschiedene Verfahren implementiert, insbesondere zur deterministischen und Bayes'schen Optimierung. Die Simulationsergebnisse können vielfältig visualisiert werden. Im Vortrag werden diese Entwicklungen vorgestellt. Weiterhin wird die erfolgreiche Zusammenarbeit mit dem Kooperationspartner im Rückblick skizziert.

Veranstalter
WIAS Berlin
Mittwoch, 06.12.2017, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Joachim Rehberg, WIAS Berlin:
On maximal parabolic regularity for non-autonomous parabolic operators, Part 1
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Weitere Informationen
Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
Mittwoch, 06.12.2017, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Dr. Alexander Meister, Universität Rostock:
Nonparametric density estimation for intentionally corrupted functional data
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
We consider statistical models where, in order to satisfy privacy constraints, functional data are artificially contaminated by independent Wiener processes. We show that the corrupted observations have a Wiener density, which determines the distribution of the original functional random variables uniquely. We construct a nonparametric estimator of the functional density and study its asymptotic properties. We provide an upper bound on its mean integrated squared error which yields polynomial convergence rates, and we establish lower bounds on the minimax convergence rates which are close to the rates attained by our estimator. Our estimator requires the choice of a basis and of two smoothing parameters. We propose data-driven ways of choosing them and prove that the asymptotic quality of our estimator is not significantly affected by the empirical parameter selection. We apply our technique to a classification problem of real data and provide some numerical results. This talk is based on a joint work with A. Delaigle (University of Melbourne).

Veranstalter
WIAS Berlin
Dienstag, 05.12.2017, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Maxim Panov, Skolkovo Institute of Science and Technology, Moskow, Russia:
Consistent estimation of mixed memberships with successive projections
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
In this talk, we consider several types of accelerated randomized gradient methods: random directional search, random coordinate descent, randomized zero-order method. Using the concept of inexact oracle, we present a generic theorem on the convergence rate for all the three methods. Despite their random nature, these methods have complexity with the same dependence on the desired accuracy of the solution as deterministic accelerated gradient method. Joint work with A. Gasnikov and A. Tiurin.

Veranstalter
WIAS Berlin
Dienstag, 05.12.2017, 10:00 Uhr (WIAS-ESH)
Seminar Laserdynamik
Prof. Raimondas Čiegis, Vilnius Gediminas Technichal University, Litauen:
Numerical simulation of nonlocal delayed feedback controller for some new smart bioreactors
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
The new nonlocal delayed feedback controller is used to control the production of drugs in a simple bioreactor. This bioreactor is based on the enzymatic conversion of substrate into the required product. The dynamics of this device is described by a system of two nonstationary nonlinear diffusion-reaction equations. The control loop defines the changes of the substrate concentration delivered into the bioreactor at the external boundary of the bioreactor depending on the difference of measurements of the produced drug delivered into the body and the flux of the drug prescribed by a doctorin accordance with the therapeutic protocol. The system of PDEs is solved by using the finite difference method, the control loop parameters are defined from the analysis of stationary linearized equations. The stability of the algorithm for the inverse boundary condition is investigated. Results of computational experiments are presented and analyzed.

Veranstalter
WIAS Berlin
Donnerstag, 30.11.2017, 16:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Modelle der Photonik
Sabrina Pickartz, WIAS Berlin:
All-optical control of fiber-solitons
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Donnerstag, 30.11.2017, 14:00 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
PD Dr. Michal Pavelka, Charles University, Tschechische Republik:
42=GENERIC - Unified Hamiltonian description of solids and fluids
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
GENERIC (General equation for non-equilibrium reversible-irreversible coupling) provides evolution equations for many physical theories: fluid mechanics, kinetic theory, mechanics of solids, plasticity, non-Newtonian fluid mechanics, electromagnetic field coupled with matter, dynamics of mixtures, etc. The equations consist of a reversible Hamiltonian part, generated by a Poisson bracket and energy, and by an irreversible gradient part, generated by a dissipation potential and entropy. The purpose of this talk is to provide introduction into GENERIC and discuss some specific applications.

Veranstalter
WIAS Berlin
Mittwoch, 29.11.2017, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Pavel Gurevich, Freie Universität Berlin:
Rattling in hysteretic reaction-diffusion systems
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

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Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
Dienstag, 28.11.2017, 10:15 Uhr (WIAS-406)
Seminar Nichtlineare Optimierung und Inverse Probleme
Christian Löbbert, RWTH Aachen:
Parallel arithmetic for distributed tensors in the HT-format
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Veranstalter
WIAS Berlin
Montag, 27.11.2017, 14:00 Uhr (WIAS-ESH)
Dr. Anne Höner, Forschungsverbund Berlin e.V.:
Funding opportunities for WIAS Researchers within the European Framework Programme Horizon 2020
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Montag, 27.11.2017, 09:30 Uhr (WIAS-HVP-4.13)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Dr. Andrea Ceretani, Humboldt-Universität zu Berlin:
Anomalous diffusion with free boundaries
mehr ... Veranstaltungsort
Weierstraß-Institut, Hausvogteiplatz 11A, 10117 Berlin, 4. Etage, Raum: 4.13

Abstrakt
Recently, time fractional Stefan-like problems have been used to model anomalous diffusion with free boundaries and long memory retention. Nevertheless, the physical foundations of this usage is still unclear. We present a model for acid water neutralization with anomalous and fast diffusion. Though this problem presents short memory retention, it is a first step in deriving mathematical models for anomalous diffusion under memory effects based on commonly accepted physical laws. The problem consists in the neutralization of an acid solution in which the hydrogen ions are transported according to Cattaneo's diffusion, and we consider the specific case of a marble slab reacting with a sulphuric acid solution in a one-dimensional geometry. The mathematical problem is reduced to a hyperbolic free boundary problem where the consumption of the slab is described by a nonlinear differential equation. We prove global well-posedness and present some numerical simulations.

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Joint Research Seminar on Mathematical Optimization / Non-smooth Variational Problems and Operator Equations

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
Freitag, 24.11.2017, 14:00 Uhr (WIAS-ESH)
Halbleiterseminar
Dr. Oliver Marquardt, WIAS Berlin:
Anwendungen der kp-Theory
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Weitere Informationen
Halbleiterseminar

Veranstalter
WIAS Berlin
Mittwoch, 22.11.2017, 14:30 Uhr (WIAS-406)
FG Stochastische Systeme mit Wechselwirkung
Dr. Sergio Simonella, Technische Universität München:
Correlations in the mean field dynamics: a random walk expansion
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Veranstalter
WIAS Berlin
22. November 2017 (WIAS-ESH)
Workshop/Konferenz: Colloquium in Mathematical Statistics
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Dienstag, 21.11.2017, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Valerij Avanesov, WIAS Berlin:
Dynamics of high-dimensional covariance matrices
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
We consider the detection and localization of an abrupt break in the covariance structure of high-dimensional random data. The study proposes two novel approaches for this problem. The approaches are essentially hypothesis testing procedures which requires a proper choice of a critical level. In that regard calibration schemes, which are in turn different non-standard bootstrap procedures, are proposed. One of the approaches relies on techniques of inverse covariance matrix estimation, which is motivated by applications in neuroimaging. A limitation of the approach is a sparsity assumption crucial for precision matrix estimation which the second approach does not rely on. The description of the approaches are followed by a formal theoretical study justifying the proposed calibration schemes under mild assumptions and providing the guaranties for the break detection. Theoretical results for the first approach rely on the guaranties for inference of precision matrix procedures. Therefore, we rigorously justify adaptive inference procedures for precision matrices. All the results are obtained in a truly high- dimensional (dimensionality p _x001d_ n) finite-sample setting. The theoretical results are supported by simulation studies, most of which are inspired by either real- world neuroimaging or financial data.

Veranstalter
WIAS Berlin
Dienstag, 21.11.2017, 10:15 Uhr (WIAS-406)
Seminar Nichtlineare Optimierung und Inverse Probleme
Prof. Eric Charkaluk, Ecole Polytechnique, Frankreich:
Fatigue of metallic additive manufactured structures: What can we learn from other manufacturing processes?
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Veranstalter
WIAS Berlin
Montag, 20.11.2017, 14:00 Uhr (WIAS-ESH)
Seminar Quantitative Biomedizin
Prof. Jean-Frederic Gerbeau, Inria de Paris & Sorbonne Universités UPMC Paris 6:
Numerical methods for variability modeling and biomarkers design
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Many phenomena are modeled by deterministic differential equations, whereas the observation of these phenomena, in particular in life science, exhibit an important inter-subject variability. We will address the following question: how the model can be adapted to reflect the variability observed in a population? We will present a non-parametric and non-intrusive procedure based on offline computations of the deterministic model. The algorithm infers the probability density function of uncertain parameters from the matching of the observable statistical moments at different points in the physical domain. This inverse procedure is improved by incorporating a point selection algorithm that both reduces its computational cost and increases its robustness. The method will be illustrated for different models, based on Ordinary or Partial Differential Equations. In particular, applications to experimental data sets in cardiac electrophysiology will be presented.
In biophysics and medicine, the system of interest is often studied by monitoring quantities, called biomarkers, extracted from measurements. These biomarkers convey some information about relevant hidden quantities, which can be seen as parameters of an underlying model. We propose a strategy to automatically design biomarkers to estimate a given parameter. Such biomarkers are chosen as the solution of a sparse optimization problem. We will show applications in electrophysiology where our algorithm provides composite biomarkers which improve the parameter estimation and the classification problems.

Veranstalter
WIAS Berlin
Donnerstag, 16.11.2017, 16:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Modelle der Photonik
Dr. Carsten Brée, WIAS Berlin:
Shaping beams of broad area lasers with photonic crystals
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Donnerstag, 16.11.2017, 14:00 Uhr (WIAS-HVP)
Seminar Materialmodellierung
Prof. Andreas Münch, University of Oxford, GB:
Asymptotic analysis of models involving surface diffusion
mehr ... Veranstaltungsort
Weierstraß-Institut, Hausvogteiplatz 11A, 10117 Berlin, 4. Etage, Raum: 4.01

Abstrakt
We study the evolution of solid surfaces and pattern formation by surface diffusion. Phase field models with degenerate mobilities are frequently used to model such phenomena, and are validated by investigating their sharp interface limits. We demonstrate by a careful asymptotic analysis involving the matching of exponential terms that a certain combination of degenerate mobility and a double well potential leads to a combination of surface and nonlinear bulk diffusion to leading order. We also present a stability analysis for the sharp interface model of an evolving non-homogeneous base state and show how to correctly determine the dominant mode, which is not the one predicted by a frozen mode eigenvalue analysis.

Veranstalter
WIAS Berlin
Mittwoch, 15.11.2017, 15:15 Uhr (WIAS-ESH)
Halbleiterseminar
Prof. Anton Boitsev, St. Petersburg National University of Information Technologies, Mechanics and Optics, RUSSIA:
Boundary triplets, tensor products and point contacts to reservoirs
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

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WIAS-Halbleiterseminar

Veranstalter
WIAS Berlin
Mittwoch, 15.11.2017, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Enkelejd Hashorva, University of Lausanne, Switzerland:
From classical to parisian ruin in Gaussian risk models
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
This talk is concerned with Gaussian risk models which approximate reasonably the risk process of an insurance company. Such models incorporate various nancial elements related to inflation/deflationion and taxation. Of interest also from the probabilistic point of view, is the approximation of the ruin probability and the ruin time when the initial capital is large. The concept of Parisian ruin is quite new and appealing for mathematical models of insurance risks. However the calculation of Parisian ruin and the Parisian ruin time is a hard problem. Recent research has also focused on the investigation of multi-valued risk models analysing the ruin probability and the ruin time. Currently, due to the lack of appropriate tools, results are available only for the Brownian risk model. In this talk various approxi- mations of ruin probability and ruin times for both classical and Parisian case will be discussed including results for the multi-valued Brownian risk model. Joint work with K. Debicki, University of Wroclaw and L. Ji, University of Lausanne

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Dienstag, 14.11.2017, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Adrien Barrasso, École Nationale Supérieure de Techniques Avancées, Paris, France:
BSDEs and decoupled mild solutions of (possibly singular, path dependent or integro-PDEs.)
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Veranstalter
WIAS Berlin
Freitag, 10.11.2017, 14:00 Uhr (WIAS-ESH)
Halbleiterseminar
Dr. Thomas Koprucki, WIAS Berlin:
Einführung in die kp-Theory
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Weitere Informationen
Halbleiterseminar

Veranstalter
WIAS Berlin
Mittwoch, 08.11.2017, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Robert Lasarzik, Technische Universität Berlin:
Generalised solutions to the Ericksen--Leslie model describing liquid crystal flow
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
here

Weitere Informationen
Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
Mittwoch, 08.11.2017, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Arnak Dalayan, ENSAE ParisTech:
On the exponentially weighted aggregate with the Laplace prior
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
In this talk, we will present some results on the statistical behaviour of the Exponentially Weighted Aggregate (EWA) in the problem of high-dimensional regression with xed design. Under the assumption that the underlying regression vector is sparse, it is reasonable to use the Laplace distribution as a prior. The resulting estimator and, speci cally, a particular instance of it referred to as the Bayesian lasso, was already used in the statistical literature because of its computational convenience, even though no thorough mathematical analysis of its statistical properties was carried out. The results of this talk ll this gap by establishing sharp oracle inequalities for the EWA with the Laplace prior. These inequalities show that if the temperature parameter is small, the EWA with the Laplace prior satis es the same type of oracle inequality as the lasso estimator does, as long as the quality of estimation is measured by the prediction loss. Extensions of the proposed methodology to the problem of prediction with low-rank matrices will be discussed as well. (based on a joint work with Edwin Grappin and Quentin Paris)

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Dienstag, 07.11.2017, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Egor Klochkov, Humboldt-Universität zu Berlin:
Invertibility of 1D random kernel matrix
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Veranstalter
WIAS Berlin
Dienstag, 07.11.2017, 13:30 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Prof. Julia Novo, Universidad Autónoma de Madrid, Spanien:
Quasi-optimal methods to approximate the incompressible Navier--Stokes squations
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
In this talk we will consider different methods to approximate the incompressible Navier--Stokes squations. Our main concern is to get error bounds with constants independent on inverse powers of the viscosity parameter. We will consider inf-sup stable methods with grad-div stabilization, non inf-sup stable methods with local projection stabilization and fully discrete schemes based on projection methods in time plus grad-div stabilization in space. Some numerical experiments will be shown to check the optimality of the theoretical rates of convergence of the methods.

Veranstalter
WIAS Berlin
Freitag, 03.11.2017, 14:00 Uhr (WIAS-ESH)
Halbleiterseminar
Dr. Manuel Landstorfer, WIAS Berlin:
Modellierung von Elektrolyt-Metall-Grenzflächen
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Weitere Informationen
Halbleiterseminar

Veranstalter
WIAS Berlin
Mittwoch, 01.11.2017, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Tomáš Roubíček, Czech Academy of Sciences, Tschechische Republik:
Seismic waves and earthquakes in a global monolithic model
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
A model of seismic sources during rupturing of lithospheric faults is combined with a model of propagation of the emitted seismic waves in a layered continuum, admitting also a viscoelastic (Boger) fluid and reflecting thus the Earth mantle/core structure. This model is approximated by a monolithic way, handling the damageable solid (with the phase-field crack model) and the Boger fluid in a unified way. Numerical simulations showing rupture on pre-existing fault, wave propagation and S-wave reflection performed by Roman Vodicka is shown, together with another experiment where a new fault and a surface depression are born.

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Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
Mittwoch, 01.11.2017, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Denis Belomestny, Universität Duisburg-Essen:
Statistical inference for McKean-Vlasov-SDEs
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
McKean-Vlasov-SDEs provide a very rich modelling framework for large complex systems. They naturally appear in modelling and simulation of turbulent flows by fluid-particle method. In biomathematics, a McKean-Vlasov-SDE model for neuronal networks has been proposed. Although potentially very powerful, the lack of efficient statistical procedures prevents further expansion of these results into application areas. When proposing a McKean-Vlasov-SDE model, one of the main challenges is the appropriate choice of the coefficients. In this talk, we study the problem of the nonparametric estimation of the McKean-Vlasov diffusion coefficients from low-frequency observations.

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Montag, 30.10.2017, 14:00 Uhr (WIAS-ESH)
INSTITUTSKOLLOQUIUM
Prof. Dr. Leonid Berlyand, Pennsylvania State University, USA:
Hierarchy of PDE Models of Cell Motility
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
We consider mathematical PDE models of motility of eukaryotic cells on a substrate and discuss them in a broader context of active materials. Our goal is to capture mathematically the key biological phenomena such as steady motion with no external stimuli and spontaneous breaking of symmetry. We first describe the hierarchy of PDE models of cell motility and then focus on two specific models: the phase-field model and the free boundary problem model. The phase-field model consists of the Allen-Cahn equation for the scalar phase field function coupled with a vectorial parabolic equation for the orientation of the actin filament network. The key mathematical properties of this system are (i) the presence of gradients in the coupling terms and (ii) the mass (volume) preservation constraints. These properties lead to mathematical challenges that are specific to active (out of equilibrium) systems, e.g., the fact that variational principles do not apply. Therefore, standard techniques based on maximum principle and Gamma-convergence cannot be used, and one has to develop alternative asymptotic techniques. The free boundary problem model consists of an elliptic equation describing the flow of the cytoskeleton gel coupled with a convection-diffusion PDE for the density of myosin motors. This PDE system is of Keller-Segel type but in a free boundary setting with nonlocal condition that involves boundary curvature. Analysis of this system allows for a reduction to a Liouville type equation which arises in various applications ranging from geometry to chemotaxis. This equation contains an additional term that presents an additional challenge in analysis. In the analysis of the above models our focus is on establishing the traveling wave solutions that are the signature of the cell motility. We also study breaking of symmetry by proving existence of non-radial steady states. Bifurcation of traveling waves from steady states is established via the Schauder's fixed point theorem for the phase field model and the Leray-Schauder degree theory for the free boundary problem model. These results are obtained in collaboration with Jan Fuhrmann, M. Potomkin, and V. Rybalko.

Veranstalter
WIAS Berlin
Mittwoch, 25.10.2017, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Joachim Rehberg, WIAS Berlin:
Explicit and uniform resolvent estimates for second order divergence operators on Lp spaces
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Weitere Informationen
Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
Mittwoch, 25.10.2017, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Debarghya Ghoshdastidar, Universität Tübingen:
Two-sample hypothesis testing for inhomogeneous random graphs
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
In this work, we consider the problem of testing between two populations of inhomogeneous random graphs de ned on the same set of vertices. We are particularly interested in the high-dimensional setting where the population size is potentially much smaller than the graph size, and may even be constant. It is known that this setting cannot be tackled if the separation between two models is quanti ed in terms of total variation distance. Hence, we study two-sample testing problems where the separation between models is quanti ed by the Frobenius or operator norms of the di erence between the population adjacency matrices. We derive upper and lower bounds for the minimax separation rate for these problems. Interestingly, the proposed near-optimal tests are uniformly consistent in both the large graph, small sample and small graph, large sample regimes. This is a joint work with Maurilio Gutzeit, Alexandra Carpentier and Ulrike von Luxburg.

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Dienstag, 24.10.2017, 15:15 Uhr (FU-140)
Oberseminar Nonlinear Dynamics
Carlos Rocha, Instituto Superior Técnico of Lisbon:
Global attractors for non-autonomous systems
mehr ... Veranstaltungsort
Freie Universität Berlin, Arnimallee 7, 14195 Berlin, Hinterhaus, Raum: 140

Abstrakt
We survey the notions of global attractors for non-autonomous systems, and consider small non-autonomous perturbations of dynamical systems to discuss the resulting changes on the global attractors. We review the notion of Morse-Smale dynamical system and extend this notion to the non-autonomous framework, based on a recent joint work with R. Czaja and W. Oliva.

Veranstalter
Freie Universität Berlin
WIAS Berlin
Dienstag, 24.10.2017, 13:45 Uhr (WIAS-406)
Seminar Materialmodellierung
Anna Zubkova, Karl-Franzens-Universität Graz, Österreich:
Homogenization of the generalized Poisson--Nernst--Planck system with nonlinear interface conditions
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
We consider the generalized system of nonlinear Poisson-Nernst-Planck equations, which describes concentrations of multiple charged particles with the overall electrostatic potential. It is modeled in terms of the Fickian multiphase diffusion law coupled with thermodynamic principles. The generalized model is supplied by volume and positivity constraints and quasi-Fermi electrochemical potentials depending on the pressure. The model describes a plenty of electrokinetic phenomena in physical and biological sciences. We examine nonlinear inhomogeneous transmission conditions describing electro-chemical reactions on the interface in a periodic two-phase medium. We aim at a proper variational modeling, well-posedness, and asymptotic analysis as well as homogenization of the model.

Veranstalter
WIAS Berlin
Donnerstag, 19.10.2017, 14:00 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Prof. Leo Rebholz, Clemson University, USA:
On conservation laws of Navier--Stokes Galerkin discretizations
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
We study conservation properties of Galerkin methods for the incompressible Navier-Stokes equations, without the divergence constraint strongly enforced. In typical discretizations such as the mixed finite element method, the conservation of mass is enforced only weakly, and this leads to discrete solutions which may not conserve energy, momentum, angular momentum, helicity, or vorticity, even though the physics of the Navier-Stokes equations dictate that they should. We aim in this work to construct discrete formulations that conserve as many physical laws as possible without utilizing a strong enforcement of the divergence constraint, and doing so leads us to a new formulation that conserves each of energy, momentum, angular momentum, enstrophy in 2D, helicity and vorticity (for reference, the usual convective formulation does not conserve most of these quantities). Several numerical experiments are performed, which verify the theory and test the new formulation.

Veranstalter
WIAS Berlin
Mittwoch, 18.10.2017, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Dr. Vladimir Spokoiny, WIAS Berlin:
Big ball probability with applications in statistical inference
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
We derive the bounds on the Kolmogorov distance between probabilities of two Gaussian elements to hit a ball in a Hilbert space. The key property of these bounds is that they are dimensional-free and depend on the nuclear (Schatten-one) norm of the difference between the covariance operators of the elements. We are also interested in the anticoncentration bound for a squared norm of a non-centered Gaussian element in a Hilbert space. All bounds are sharp and cannot be improved in general. We provide a list of motivation examples and applications in statistical inference for the derived results as well. (joint with Götze, Naumov and Ulyanov)

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
Universität Potsdam
Dienstag, 17.10.2017, 15:15 Uhr (FU-140)
Oberseminar Nonlinear Dynamics
Dr. Jan Sieber, University of Exeter, UK:
Smooth center manifolds for delay-differential equations
mehr ... Veranstaltungsort
Freie Universität Berlin, Arnimallee 7, 14195 Berlin, Hinterhaus, Raum: 140

Abstrakt
Delay-differential equations (DDEs) with state-dependent delays are, as far as is known, at best continuously differentiable once as dynamical systems. That is, the time-t map does not depend on its argument with a higher degree of smoothness than 1. However, as I will show, center manifolds near equilibria are still as smooth as expected from the spectral gap and from the smoothness of coefficients. In particular, I will review what precisely ßmoothness of coefficients" means.

Veranstalter
Freie Universität Berlin
WIAS Berlin
Dienstag, 17.10.2017, 15:00 Uhr (WIAS-ESH)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Alexandra Suvorikova, WIAS Berlin:
Two-sample test based on 2-Wasserstein distance
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Montag, 16.10.2017, 14:00 Uhr (WIAS-ESH)
INSTITUTSKOLLOQUIUM
Dr. Shalva Amiranashvili, WIAS:
Negative frequency radiation in optical fibers
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Donnerstag, 12.10.2017, 10:30 Uhr (HU Berlin in Adlershof RUD25 Raum 2.417)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Dr. Stephan Schmidt, Universität Würzburg:
SQP methods for shape optimization based on weak shape Hessians
mehr ... Veranstaltungsort
Humboldt-Universität zu Berlin, Rudower Chaussee 25, 12489 Berlin, Johann-von-Neumann-Haus, Raum 2.417

Abstrakt
Many PDE constrained optimization problems fall into the category of shape optimization, meaning the geometry of the domain is the unknown to be found. Most natural applications are drag minimization in fluid dynamics, but many tomography and image reconstruction problems also fall into this category. The talk introduces shape optimization as a special sub-class of PDE constraint optimization problems. The main focus here will be on generating Newton-like methods for large scale applications. The key for this endeavor is the derivation of the shape Hessian, that is the second directional derivative of a cost functional with respect to geometry changes in a weak form based on material derivatives instead of classical local shape derivatives. To avoid human errors, a computer aided derivation system is also introduced. The methodologies are tested on problem from fluid dynamics and geometric inverse problems.

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Joint Research Seminar on Mathematical Optimization / Non-smooth Variational Problems and Operator Equations

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
11. – 14. Oktober 2017 (WIAS-ESH)
Workshop/Konferenz: SPP 1962 Autumn School 2017 “Nonsmooth Structures in Mathematical Models”
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
There will be an autumn school on "Nonsmooth Structures in Mathematical Models" at the Weierstrass Institute in Berlin as part of the DFG SPP 1962. We will have three lecturer courses: 'Splitting algorithms in nonsmooth convex and nonconvex optimization' by Prof. Dr. Bot, 'Systems with rate independence: models, sensitivity, optimization' by Prof. Dr. Brokate, 'Problems with equilibrium constraints: Theory and numerics' by Prof. Dr. Outrata. More details of the program can be found at http://www.wias-berlin.de/workshops/sppsummerschool2017/program.jsp. The autumn school will start at 4pm on the 11th October and end around 12:30pm on the 14th October. This school is primarily intended for the doctoral and postdoctoral students. Interested parties may apply to register; the number of available places is limited so registration is mandatory for all and the deadline for registration is 30th August. There is no registration fee, however travel costs will not be paid. For full details and registration, please see http://www.wias-berlin.de/workshops/sppsummerschool2017/index.jsp.

Veranstalter
WIAS Berlin
DFG Schwerpunktprogramm 1962
4. – 6. Oktober 2017 (WIAS-ESH)
Workshop/Konferenz: Homogenization Theory and Applications
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
SFB 1114: Scaling Cascades in Complex Systems
SFB 910 “Control of self-organizing nonlinear systems: Theoretical methods and concepts of application”
Dienstag, 26.09.2017, 13:30 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Yuxue Ren, Dalian University of Technology, China:
On tetrahedralisations containing knotted and linked line segments
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
This talk considers a set of twisted line segments in 3d such that they form a knot (a closed curve) or a link of two closed curves. Such line segments appear on the boundary of a family of 3d indecomposable polyhedra (like the Schönhardt polyhedron) whose interior cannot be tetrahedralised without additional vertices added. On the other hand, a 3d (non-convex) polyhedron whose boundary contains such line segments may still be decomposable as long as the twist is not too large. It is therefore interesting to consider the question: when there exists a tetrahedralisation contains a given set of knotted or linked line segments?
In this talk, we studied a simplified question with the assumption that all vertices of the line segments are in convex position. It is straightforward to show that no tetrahedralisation of 6 vertices (the three-line-segments case) can contain a trefoil knot. When the number of twisted line segments is larger than 3, it is necessary to create new interior edges to form a tetrahedralisation. We provided a detailed analysis for the case of a set of 4 twisted line segments. We show that the addition of a pair of new interior edges decomposes the original knot (or link) into two links (or knots) with less crossing numbers. This leads to a crucial condition on the orientation of pairs of new interior edges which determines whether this set is decomposable or not. We then prove a new theorem about the decomposability for a set of n (n ≥ 3) twisted line segments. This theorem implies that the family of polyhedra generalised from the Schönhardt polyhedron by Rambau are all indecomposable.

Veranstalter
WIAS Berlin
Donnerstag, 21.09.2017, 14:00 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Dr. Christoph Freysoldt, Max-Planck-Institut für Eisenforschung, Düsseldorf:
Vortrag entfällt / Lecture not applicable
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Dienstag, 19.09.2017, 15:00 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Matteo Patriarca, University of Rome “Tor Vergata”, Italy:
Discontinuous finite element discretization for semiclassical particle transport
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
In order to understand how semiconductor devices operate, it is very common to solve a Poisson equation coupled with drift-diffusion equations for electrons and holes. We present several discontinuous approaches to solving the Poisson equation: non-symmetric interior penalty, lifting operator and a mixed approach among finite volume method and discontinuous Galërkin finite element method. We discuss the advantages and drawbacks of these approaches in order to extend the best discretization method to the drift-diffusion model. The stationary case is implemented in our software simulator TiberCAD. Simulation results are shown to highlight the problems encountered.

Veranstalter
WIAS Berlin
Dienstag, 19.09.2017, 10:15 Uhr (WIAS-ESH)
Seminar Nichtlineare Optimierung und Inverse Probleme
Sebastian Wolf, Technische Universität Berlin:
Tensor reconstruction
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Montag, 18.09.2017, 14:00 Uhr (WIAS-ESH)
Seminar Quantitative Biomedizin
Prof. Dominique Chapelle, Inria Saclay -- Ile de France , Frankreich:
Biomechanical modeling of the heart, and cardiovascular system --- From sarcomeres to organ/system, with experimental assessments and patient-specific clinical validations
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Cardiac contraction originates at a subcellular - molecular, indeed - scale, within specific components of the cardiomyocytes (i.e. cardiac cells) called sarcomeres. This contractile behaviour then needs to be integrated at the organ level, namely, with a specific structure and shape. Furthermore, this organ crucially interacts with other physiological systems, the first of which being blood circulation via the cardiac function itself, and also the nervous system that controls the heart via various regulation mechanisms, and these interactions must be adequately represented in order to obtain accurate and predictive model simulations. This presentation will provide an overview of the recent advances on cardiac modeling achieved in the M3DISIM group, with a particular focus on the key multiscale, multi-physics and integrated system modeling aspects that need to be addressed, with many associated challenges pertaining to numerical methods, model validation, and inverse problems, in particular.

Veranstalter
WIAS Berlin
Donnerstag, 14.09.2017, 15:00 Uhr (WIAS-405-406)
Seminar Numerische Mathematik
Prof. Na Lei, Dalian University of Technology, China:
Quadrilateral and hexahedral mesh generation based on surface foliation theory
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Abstrakt
For the purpose of isogeometric analysis, one of the most common ways is to construct structured hexahedral meshes, which have regular tensor product structure, and fit them by volumetric T-Splines. This theoretic work proposes a novel surface quadrilateral meshing method, colorable quad-mesh, which leads to the structured hexahedral mesh of the enclosed volume for high genus surfaces. The work proves the equivalence relations among colorable quad-meshes, finite measured foliations and Strebel differentials on surfaces. This trinity theorem lays down the theoretic foundation for quadrilateral/hexahedral mesh generation, and leads to practical, automatic algorithms. The work proposes the following algorithm: the user inputs a set of disjoint, simple loops on a high genus surface, and specifies a height parameter for each loop; a unique Strebel differential is computed with the combinatorial type and the heights prescribed by the user?s input; the Strebel differential assigns a flat metric on the surface and decomposes the surface into cylinders; a colorable quad-mesh is generated by splitting each cylinder into two quadrilaterals, followed by subdivision; the surface cylindrical decomposition is extended inward to produce a solid cylindrical decomposition of the volume; the hexahedral meshing is generated for each volumetric cylinder and then glued together to form a globally consistent hex-mesh.

Veranstalter
WIAS Berlin
Donnerstag, 14.09.2017, 14:00 Uhr (WIAS-405-406)
Seminar Numerische Mathematik
Prof. Vladimir Garanzha, Russian Academy of Sciences, Moskau:
Construction of quasi-isometric elastic deformations in mesh generation problems
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Abstrakt
Theory of quasi-isometric (QI) mappings allows to reformulate in unified terms the intuitive idea of quasi-uniform meshes and presents natural multidimensional generalization of equidistribution principle. Suprisingly theory of QI mapping is not very rich. Rigorous results about existence of QI mappings are available only in 2d, see works by Reshetnyak, Bakelman, Bonk and Lang, Burago on parameterization of 2d manifolds of bounded curvature. In 2000 Garanzha suggested stored energy with guaranteed QI minimizer. Multidimensional "existence theorems" (Garanzha, 2005, 2010) essentially follow formulations of existence theorems of finite hyperelasticity due to John Ball: minimum of stored energy is attained on admssible mapping provided that at least one admissible mapping exists. It is not known how to find this initial admissible mapping. Another unsolved problem is lack of discrete convergence: one cannot prove that solution of discretized variational problem converges to exact minimizer. In order to construct QI mappings one has to solve continuation problem which minimizes certain evaluation of quasi-isometry constant. While providing rigorous foundations and best results in terms of distortion, this aproach is too complicated for solving engineering problems. Simple solution to this problem is based on introducing spatial weight distributions which premultiply density of stored energy (distortion measure) and sharply increase in the zones where small distortion is required. Strategies for devising these weight functions are discussed as applied to construction of smooth and orthogonal boundary layer meshes with precise stretching control, to construction of prismatic mesh layer using elastic springback technique, to surface flattening and parameterization and to construction of adaptive meshes.

Veranstalter
WIAS Berlin
13. – 15. September 2017 (WIAS-ESH)
Workshop/Konferenz: Workshop on Mathematics of Deep Learning
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Technische Universität Berlin
DFG Research Center MATHEON
DFG Research Unit 1735
DFG Research Unit 2402
Donnerstag, 07.09.2017, 16:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Modelle der Photonik
NUSOD-Highlights:
Nachlese zur NUSOD 2017
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Donnerstag, 31.08.2017, 14:00 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Clemens Bartsch, WIAS Berlin:
A mixed stochastic-deterministic approach to particles interacting in a flow
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
We present a new approach to the solution of coupled particle population balance systems. Such population balance systems consist of particles transported by a moving fluid. The particles interact with each other and with the surrounding fluid. Population balance systems aim at keeping track of both particle- and fluid quantities. In our approach, particle-particle interactions and particle-fluid interactions are solved with a stochastic simulation algorithm, while a finite element flow solver is used for fluid-fluid interactions. This best-of-two-worlds approach enables the efficient treatment even of such particle-fluid systems, where the particles are described by multiple internal coordinates. A simulation of a flow crystallizer will be presented as a proof of concept.

Veranstalter
WIAS Berlin
Dienstag, 29.08.2017, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Alexander Zadorozhnyi, Universität Potsdam:
Concentration of weakly dependent Banach-valued sums and applications to kernel learning methods
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
We obtain a new Bernstein-type inequality for sums of Banach-valued random variables satisfying a weak dependence assumption of general type and under certain smoothness assumptions of the underlying Banach norm. We use this inequality in order to investigate statistical rates of convergence for the broad family of spectral regularization methods for reproducing kernel decision rules, when trained on a sample coming from a stationary $tau-$mixing process.

Veranstalter
WIAS Berlin
Mittwoch, 09.08.2017, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Dr. Dongo Chae, Chung-Ang University, Korea:
On the blow-up problem for the incompressible Euler equation
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
here

Weitere Informationen
Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
Mittwoch, 02.08.2017, 16:00 Uhr (WIAS-406)
FG Stochastische Systeme mit Wechselwirkung
Prof. Dr. Alexander Drewitz, Universität zu Köln:
The maximal particle of branching RW in random branching environment
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
We consider branching random walk in spatially random branching environment (BRWRE) in dimension one, as well as related differential equations: the Fisher-KPP equation with random branching and its linearized version the parabolic Anderson model (PAM). When the random environment is bounded, we show that after recentering and scaling, the position of the maximal particle of the BRWRE, the front of the solution of the PAM, as well as the front of the solution of the randomized Fisher-KPP equation fulfill quenched invariance principles. In addition, we prove that at time t the distance between the median of the maximal particle of the BRWRE and the front of the solution of the PAM is in O(ln t). This is joint work with Jiří Černý.

Veranstalter
WIAS Berlin
Mittwoch, 02.08.2017, 15:00 Uhr (WIAS-406)
FG Stochastische Systeme mit Wechselwirkung
Prof. Dr. Markus Kraft, University of Cambridge, GB:
Moment projection method for solving population balance equations
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
A new moment method for solving the population balance equations is presented. The moment projection method (MPM) addresses the challenge of particle shrinkage due to processes such as oxidation, evaporation or dissolution and particle fragmentation. It directly solves the moment transport equation for the moments and tracks the number of the smallest particles using the algorithm by Blumstein and Wheeler. The new method was validated for particle inception, coagulation, condensation, growth, shrinkage and fragmentation. Different types of particle size distributions (PSDs) are supplied as an initial condition. Comparisons are made with the method of moments (MOM), hybrid method of moments, quadrature method of moments and a high-precision stochastic solution calculated using the established direct simulation algorithm (DSA). The results suggest that MPM shows improved performance compared to MOM, HMOM and QMOM in the treatment of shrinkage and fragmentation. The new method predicts mean quantities which are almost as accurate as the stochastic solution.

Veranstalter
WIAS Berlin
Mittwoch, 02.08.2017, 14:00 Uhr (WIAS-406)
FG Stochastische Systeme mit Wechselwirkung
Prof. Dr. Orazio Muscato, Università degli Studi di Catania, Italien:
The evergreen Wigner transport equation
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Veranstalter
WIAS Berlin
Mittwoch, 02.08.2017, 14:00 Uhr (WIAS-406)
FG Stochastische Systeme mit Wechselwirkung
Orazio Muscato, Università degli Studi di Catania, Italien:
The evergreen Wigner transport equation
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Veranstalter
WIAS Berlin
Dienstag, 01.08.2017, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Andzhey Koziuk, WIAS (Berlin):
Smooth representation of the Kolmogorov distance
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
In this talk, we consider several types of accelerated randomized gradient methods: random directional search, random coordinate descent, randomized zero-order method. Using the concept of inexact oracle, we present a generic theorem on the convergence rate for all the three methods. Despite their random nature, these methods have complexity with the same dependence on the desired accuracy of the solution as deterministic accelerated gradient method. Joint work with A. Gasnikov

Veranstalter
WIAS Berlin
Mittwoch, 26.07.2017, 13:15 Uhr (WIAS-ESH)
Seminar Nichtglatte Variationsprobleme und Operatorgleichungen
Prof. Hasnaa Zidani, ENSTA ParisTech, Frankreich:
Multi-objective control problems under state constraints
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Dienstag, 25.07.2017, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Benjamin Stemper, WIAS Berlin:
Some thoughts on calibration under rough volatility via deep learning
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Veranstalter
WIAS Berlin
24. Juli 2017 (WIAS-ESH)
Workshop/Konferenz: Kolloquium zum Ruhestand von Dr. G. Schmidt
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Donnerstag, 20.07.2017, 16:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Modelle der Photonik
Sabrina Pickartz, WIAS Berlin:
Cancellation of Raman self-frequency shift for compression of optical pulses
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Mittwoch, 19.07.2017, 18:15 Uhr (WIAS-ESH)
Berliner Kolloquium Wahrscheinlichkeitstheorie/Berlin Young Probabilists' Seminar
Prof. Dr. Anton Bovier, Rheinische Friedrich-Wilhelms-Universität Bonn:
Adaptive dynamics, diploid models, and the escape from selection
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Standard stochastic models of adaptive dynamics are based on haploid reproduction schemes. They are sometimes criticised for the difficulty to create genetic diversity. This can be traced to the fact that unless parameters are fine-tuned, times of fixation and extinctions are of the same order. We show that in diploid models, it fitter alleles are dominant, extinction times are dramatically increased. We show that, under reasonable assumptions, this can lead to the recovery of an initial unfit allele after the appearance of a second mutant allele, leading to the coexistence of two phenotypes without fin-tuning hypothesis. (joint work with Rebecca Neukirch and Loren Coquille)

Weitere Informationen
Berliner Kolloquium Wahrscheinlichkeitstheorie

Veranstalter
Humboldt-Universität zu Berlin
Technische Universität Berlin
Universität Potsdam
WIAS Berlin
Dienstag, 18.07.2017, 15:15 Uhr (WIAS-ESH)
Oberseminar Nonlinear Dynamics
Jia-Yuan Dai, Freie Universität Berlin:
Spiral waves in circular and spherical geometries. The Ginzburg--Landau paradigm
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
We prove the existence of spiral waves for the complex Ginzburg-Landau equation in the circular and spherical geometries. Instead of applying the shooting method from the literature, we establish a functional approach and adopt global bifurcation analysis to solve the spiral wave elliptic equation. Moreover, we prove the existence of two new patterns: frozen spirals in the circular and spherical geometries and spiral-pairs, that is, spirals with two tips, in the spherical geometry.

Veranstalter
Freie Universität Berlin
WIAS Berlin
Dienstag, 18.07.2017, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Mario Maurelli, WIAS Berlin:
2D Stochastic Euler equations: Flow representation and vortex approximation
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
We consider two dimensional Euler equations (in vorticity form) for incompressible fluids, with a stochastic term of transport-type. Deterministic Euler equations in 2D can be associated with an ODE with non-local (mean field) way. We extend this interpretation to the stochastic case and we provide well-posedness and suitable particle approximation. This is based on joint works with Michele Coghi, Franco Flandoli and Zdzislaw Brzezniak.

Veranstalter
WIAS Berlin
Dienstag, 18.07.2017, 13:30 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Dr. Dirk Peschka, WIAS Berlin:
Variational structure of viscous flows with contact line motion
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
In this talk I will present variational approach to the free boundary problem of fluid motion over solid and liquid substrates, focussing on algorithmic approaches for the treatment of contact line motion.

Veranstalter
WIAS Berlin
Donnerstag, 13.07.2017, 16:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Modelle der Photonik
Markus Kantner, WIAS Berlin:
Hybrid quantum-classical modeling of quantum dot devices
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Mittwoch, 12.07.2017, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar) & Seminar Materialmodellierung
Prof. Dr. Rodica Toader, University of Udine, Italy:
Existence for dynamic Griffith fracture with a weak maximal dissipation condition
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
The study of dynamic fracture is based on the dynamic energy-dissipation balance. This condition is always satisfied by a stationary crack together with a displacement satisfying the system of elastodynamics. Therefore to predict crack growth a further principle is needed. We introduce a weak maximal dissipation condition that, together with elastodynamics and energy balance, provides a model for dynamic fracture, at least within a certain class of possible crack evolutions. In particular, we prove the existence of dynamic fracture evolutions satisfying this condition, subject to smoothness constraints, and exhibit an explicit example to show that maximal dissipation can indeed rule out stationary cracks. These results are obtained in collaboration with G. Dal Maso (SISSA) and C. Larsen (WPI).

Weitere Informationen
Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar) & Seminar Materialmodellierung

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
Mittwoch, 12.07.2017, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Wolfgang Polonik, University of California at Davis, USA:
Statistical topological data analysis: Rescaling the persistence diagram
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
A persistence diagram (PD) is one of the basic objects underlying topological data analysis. It is used to analyze topological and geometric features of an underlying space M, assuming availability of a random sample from M. Existing approaches for such analyses will be reviewed brie y, and their bene ts and shortcomings will be discussed. Then we introduce ideas for rescaling PDs, which enables the derivation of novel limit theorems for the total k persistence, and other functionals of PDs. The long-term goal of studying the rescaling of PDs is to develop novel types of statistical analysis of persistence diagrams.

Weitere Informationen
Forschungsseminar “Mathematische Statistik”

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Dienstag, 11.07.2017, 10:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Chiheb Ben Hammouda, King Abdullah University of Science and Technology - KAUST, Saudi-Arabien, Königreich:
Multilevel hybrid split-step implicit tau-leap
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
In biochemically reactive systems with small copy numbers of one or more reactant molecules, the dynamics is dominated by stochastic effects. To approximate those systems, discrete state-space and stochastic simulation approaches have been shown to be more relevant than continuous state-space and deterministic ones. In systems characterized by having simultaneously fast and slow timescales, existing discrete space-state stochastic path simulation methods, such as the stochastic simulation algorithm (SSA) and the explicit tau-leap (explicit-TL) method, can be very slow. Implicit approximations have been developed to improve numerical stability and provide efficient simulation algorithms for those systems. Tn this talk, we propose an efficient Multilevel Monte Carlo (MLMC) method in the spirit of the work by Anderson and Higham (SIAM Multiscal Model. Simul. 10(1), 2012). This method uses split-step implicit tau-leap (SSI-TL) at levels where the explicit-TL method is not applicable due to numerical stability issues. We present numerical examples that illustrate the performance of the proposed method.

Weitere Informationen
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics

Veranstalter
WIAS Berlin
Montag, 10.07.2017, 14:00 Uhr (WIAS-406)
FG Stochastische Systeme mit Wechselwirkung
Prof. Dr. Francois Baccelli, University of Texas at Austin:
Stochastic Geometry and Queuing in Wireless Networks
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
Stochastic Geometry allows one to statistically represent contention for spectrum in large wireless networks. It is based on the computation of spatial averages over a snapshot of the network state, which are not easy to interconnect with the time averages stemming from queuing dynamics. The aim of this lecture is to survey ongoing research on the mathematical interconnection of Stochastic Geometry and Queuing Theory in this wireless networking context. The survey will cover progress on the analysis of a fundamental model, which is the spatial queuing process where user service rates are determined by the interference field. It will also revisit the classical problem of multi-hop relaying queues in mobile networks, for which a new mean-field analysis will be discussed.

Veranstalter
WIAS Berlin
Mittwoch, 05.07.2017, 18:15 Uhr (TUB-MA 041)
Berliner Kolloquium Wahrscheinlichkeitstheorie/Berlin Young Probabilists' Seminar
Prof. Dr. Ilya Goldsheid, Queen Mary, University of London, GB:
Invariant Measure for Random Walks on Random Environments
mehr ... Veranstaltungsort
Technische Universität, Straße des 17. Juni 136, 10623 Berlin, MA 041

Abstrakt
It is well known that a random walk in random environment on a strip in the "environment viewed from the particle" setting is a Markov chain on the set of environments. In this context, the fundamental question is that of the existence of the density of the invariant measure of this Markov chain with respect to the measure on the set of environments. In the case of the walk on a strip, it turns out to be possible to describe all positive sub-exponentially growing solutions of the corresponding invariant measure equation in the deterministic setting and then derive the necessary and sufficient conditions for the existence of the density when the environment is stationary and ergodic. This is a joint work with D. Dolgopyat.

Weitere Informationen
Berliner Kolloquium Wahrscheinlichkeitstheorie

Veranstalter
Humboldt-Universität zu Berlin
Technische Universität Berlin
Universität Potsdam
WIAS Berlin
Dienstag, 04.07.2017, 15:15 Uhr (WIAS-ESH)
Oberseminar Nonlinear Dynamics
Augusto Visintin, Università di Trento, Italy:
Compactness and structural stability of nonlinear flows
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
Freie Universität Berlin
WIAS Berlin
Dienstag, 04.07.2017, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Lars Ruthotto, Emory University, Atlanta, USA:
Stable architectures for deep neural networks
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Veranstalter
WIAS Berlin
Donnerstag, 29.06.2017, 16:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Modelle der Photonik
Dr. Alexander Pimenov, WIAS Berlin:
Efficient simulation and analysis of multi-mode laser dynamics using time-delay models
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Mittwoch, 28.06.2017, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Bernd Sturmfels, Max Planck Institute for Mathematics in the Sciences Leipzig, Germany:
Geometry of log-concave density estimation
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
We present recent work with Elina Robeva and Caroline Uhler that establishes a new link between geometric combinatorics and nonparametric statistics. It concerns shape-constrained densities on d-space that are log-concave, with focus on the maximum likelihood estimator (MLE) for weighted samples. Cule, Samworth, and Stewart showed that the logarithm of the optimal log-concave density is piecewise linear and supported on a regular subdivision of the samples. This defines a map from the space of weights to the set of regular subdivisions of the samples, i.e. the face poset of their secondary polytope. We prove that this map is surjective. In fact, every regular subdivision arises in the MLE for some set of weights with positive probability, but coarser subdivisions appear to be more likely to arise than finer ones. To quantify these results, we introduce a continuous version of the secondary polytope, whose dual we name the Samworth body.

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
Universität Potsdam
Dienstag, 27.06.2017, 15:15 Uhr (WIAS-ESH)
Oberseminar Nonlinear Dynamics
Igor Franovic, University of Belgrade, Serbia:
Bistability, rate oscillations and slow rate fluctuations in networks of noisy neurons with coupling delay
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Spontaneous activity of cortical neurons is typically characterized as a doubly-stochastic process, underlying two distinct forms of variability. While the local spike-train variability is reflected on the fast timescale, the variability associated with much longer timescales involves macroscopic irregular fluctuations of the firing rate. The latter fluctuations apparently emerge by coherent switching of neurons between the ?up? and ?down? states of membrane potential, and are believed to play important functional roles. In order to gain qualitative insight into the mechanisms behind such switching phenomena, we consider a random network of rate-based neurons influenced by external and internal noise, as well as the coupling delay. The network behavior is analyzed by deriving the second-order stochastic mean-field model, which describes the network dynamics in terms of the mean-rate and the associated variance. The mean-field model is used to study the stability and bifurcations in the thermodynamic limit, as well as the fluctuations due to the finite-size effect. For the thermodynamic limit, it is established that (i) the network may exhibit coexistence between two stationary levels in a wide range of parameters, whereby the two types of noise affect the levels in a fundamentally different fashion, and (ii) coupling delay may give rise to oscillations of the mean-rate. The slow rate fluctuations are demonstrated to emerge via two distinct scenarios. In the delay-free case, the leading mechanism can be seen as noise-induced transitions between two metastable states, quite reminiscent to fluctuations of a particle in a double-well potential. In the second scenario, which involves the cooperative action of noise and delay, the fluctuations can be interpreted as stochastic mixing between two different oscillatory regimes.

Weitere Informationen
Oberseminar Nonlinear Dynamics

Veranstalter
Freie Universität Berlin
WIAS Berlin
Dienstag, 27.06.2017, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Cassandra Uebel, Humboldt-universtiät zu Berlin:
Variable selection with hamming loss
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Veranstalter
WIAS Berlin
Dienstag, 27.06.2017, 10:15 Uhr (WIAS-ESH)
Seminar Nichtlineare Optimierung und Inverse Probleme
Markus Mäck, Universität Stuttgart:
Numerical implementation of Fuzzy arithmetic in uncertainty analysis
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Montag, 26.06.2017, 15:15 Uhr (WIAS-ESH)
Seminar Nichtlineare Optimierung und Inverse Probleme
Prof. Dr. Michael Hanss, Universität Stuttgart:
Fuzzy arithmetic and probability theory in uncertainty analysis -- Unity in diversity
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Donnerstag, 22.06.2017, 16:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Modelle der Photonik
Dr. Carsten Brée, WIAS:
Feedback-operator for a photonic crystal in the external cavity of a broad area laser diode
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Donnerstag, 22.06.2017, 10:00 Uhr (WIAS-ESH)
Seminar Laserdynamik
Sebastian Eydam, WIAS Berlin:
Phase oscillator mode-locking
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Weitere Informationen
Seminar Laserdynamik

Veranstalter
WIAS Berlin
Donnerstag, 22.06.2017, 09:30 Uhr (HU Berlin in Adlershof RUD25 Raum 2.417)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Caroline Löbhard, WIAS:
An adaptive space-time discretization for parabolic optimal control problem with state constraints
mehr ... Veranstaltungsort
Humboldt-Universität zu Berlin, Rudower Chaussee 25, 12489 Berlin, Johann-von-Neumann-Haus, Raum 2.417

Abstrakt
We present a space-time discretization which is based on a reformulation of the stationarity conditions of a Moreau-Yosida regularized parabolic optimal control problem, which involves only the state variable. The resulting nonlinear partial differential equation is of fourth order in space, and of second order in time. In order to cope with the disbalance of regularity of the respective solutions, we develop a taylored discontinuous Galerkin scheme and derive convergence rates in the mesh size as well as an integrated update strategy for the regularization parameter related to the state constraints. We also propose an adaptive mesh refinement strategy and illustrate the performance of our method in numerical test cases.

Weitere Informationen
Joint Research Seminar on Mathematical Optimization / Non-smooth Variational Problems and Operator Equations

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Mittwoch, 21.06.2017, 15:30 Uhr (WIAS-HVP-4.13)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Mattia Bongini, Barcelona Graduate School of Economics:
Optimal Control Problems in Transport Dynamics
mehr ... Veranstaltungsort
Weierstraß-Institut, Hausvogteiplatz 11A, 10117 Berlin, 4. Etage, Raum: 4.13

Abstrakt
In this talk we shall discuss several results concerning the "indirect control of populations", i.e., how to influence a group of individuals by means of external agents with a directly controlled dynamics. By using the general theory of functionals defined on spaces of measures, we give sufficient conditions for the existence of optimal control strategies and then we present a Pontryagin Maximum Principle for such controls in the form of an Hamiltonian flow in the Wasserstein space of probability measure. Finally, we present an application of the above framework to the evacuation problem of a crowd from an unknown environment with the help of undercover stewards.

Veranstalter
WIAS Berlin
Mittwoch, 21.06.2017, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Dr. Jan Prüß, Martin-Luther-Universität Halle-Wittenberg:
Critical spaces for quasilinear evolution equations and applications
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
In the last decades, considerable effort in pde's has been put into finding the most general spaces of Initial data such that a given nonlinear pde is well-posed. So far, this has been achieved with case studies, each equation has ist own theory. Moreover, the available proofs in the literature are usually arkwardly involved, sometimes going even into the miscroscopic theory of the relevant spaces, in most cases Besov spaces. In my talk, I present an abstract approach in the framework of parabolic evolution equations relying on maximal Lp-regularity in time weighted Lp-spaces. This approach easily recovers many known results in applications, for example for the Navier-Stokes equation, the Keller-Segal equations, quasi-geostrophic equations, and many others, but also leads in a direct, smooth way to new results, e.g. for the vorticity equations.

Weitere Informationen
Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
Mittwoch, 21.06.2017, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Denis Chetverikov, University of California, Los Angeles, USA:
On cross-validated lasso
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
In this paper, we derive a rate of convergence of the Lasso estimator when the penalty parameter $lambda$ for the estimator is chosen using $K$-fold cross-validation; in particular, we show that in the model with the Gaussian noise and under fairly general assumptions on the candidate set of values of $lambda$, the prediction norm of the estimation error of the cross-validated Lasso estimator is with high probability bounded from above up to a constant by $(slog p /n)^1/2cdot log^7/8(p n)$, where $n$ is the sample size of available data, $p$ is the number of covariates, and $s$ is the number of non-zero coefficients in the model. Thus, the cross-validated Lasso estimator achieves the fastest possible rate of convergence up to a small logarithmic factor $log^7/8(p n)$. In addition, we derive a sparsity bound for the cross-validated Lasso estimator; in particular, we show that under the same conditions as above, the number of non-zero coefficients of the estimator is with high probability bounded from above up to a constant by $slog^5(p n)$. Finally, we show that our proof technique generates non-trivial bounds on the prediction norm of the estimation error of the cross-validated Lasso estimator even if the assumption of the Gaussian noise fails; in particular, the prediction norm of the estimation error is with high-probability bounded from above up to a constant by $(slog^2(p n)/n)^1/4$ under mild regularity conditions.(The authors are Denis Chetverikov, Zhipeng Liao, and Victor Chernozhukov.)

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Dienstag, 20.06.2017, 10:45 Uhr (WIAS-ESH)
Seminar Nichtlineare Optimierung und Inverse Probleme
Dimitrios Loukrezis, Technische Universität Darmstadt:
Low-rank tensor decompositions for high-dimensional uncertainty quantification in electromagnetic field problems
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Dienstag, 20.06.2017, 10:15 Uhr (WIAS-ESH)
Seminar Nichtlineare Optimierung und Inverse Probleme
Dr. Ulrich Römer, Technische Universität Darmstadt:
Stochastic collocation with adjoint error control for uncertainty quantification in computational electromagnetics
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Donnerstag, 15.06.2017, 16:00 Uhr (WIAS-HVP-3.13)
Forschungsseminar Mathematische Modelle der Photonik
Anissa Zheguzi, Ferdinand-Braun-Institut Berlin:
Simulations of broad area high-power lasers with optical feedback
mehr ... Veranstaltungsort
Weierstraß-Institut, Hausvogteiplatz 11A, 10117 Berlin, 3. Etage, Raum: 3.13

Veranstalter
WIAS Berlin
Mittwoch, 14.06.2017, 10:00 Uhr (WIAS-HVP-4.13)
Forschungsseminar Mathematische Statistik
Prof. Jean-Michel Loubes, Université Toulouse Paul Sabatier, Frankreich:
Kantorovich distance based kernel for Gaussian processes : Estimation and forecast
mehr ... Veranstaltungsort
Weierstraß-Institut, Hausvogteiplatz 11A, 10117 Berlin, 4. Etage, Raum: 4.13

Abstrakt
Monge-Kantorovich distances, otherwise known as Wasserstein distances, have received a growing attention in statistics and machine learning as a powerful discrepancy measure for probability distributions. Here, we focus on forecasting a Gaussian process indexed by probability distributions. For this, we provide a family of positive definite kernels built using transportation based distances. We provide a probabilistic understanding of these kernels and characterize the corresponding stochastic processes. We prove that the Gaussian processes indexed by distributions corresponding to these kernels can be efficiently forecast, opening new perspectives in Gaussian process modeling

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
Universität Potsdam
Dienstag, 13.06.2017, 15:00 Uhr (WIAS-HVP-4.13)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Alexandra Suvorikova, WIAS Berlin:
Detection of structural breaks in complex data
mehr ... Veranstaltungsort
Weierstraß-Institut, Hausvogteiplatz 11A, 10117 Berlin, 4. Etage, Raum: 4.13

Abstrakt
In this talk we discuss two approaches to detection of structural breaks in complex data. We focus on the online mode, where a structural break in a running stochastic process should be detected as soon as possible under constraint on a given in advance false-alarm rate. We exploit a framework, that is common for the solution of problems of this type, namely the hypothesis testing in a scrolling window. The first approach relies on the quasi likelihood-ratio test. The second part of the present work considers the case, when observed data set comes from a space of probability measures with finite second moments endowed with 2 -Wasserstein distance.

Veranstalter
WIAS Berlin
Montag, 12.06.2017, 14:00 Uhr (WIAS-ESH)
Seminar Quantitative Biomedizin
Professor Tobias Schäffter, Physikalisch-Technische Bundesanstalt Berlin:
Advances in Cardiac and Quantitative MRI
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Cardiac Magnetic Resonance (CMR) imaging has become a clinically useful tool to assess different physiological parameter in one examination. The widespread use of cardiac MRI, however, is often hampered by the complexity of MRI and the long acquisition time. For this novel, 3D MR-acquisitions have been developed to obtain anatomical, functional and flow information of the whole heart. Fast acquisition can be achieved by advanced motion compensation and new reconstruction approaches. Furthermore, a more quantitative oriented diagnosis can be achieved by pixel-wise measurement of parameters. For instance, mapping of the relaxation times allows the assessment of tissue characteristics and the estimation of contrast agent concentration. In this talk an overview on the different technical developments are given and clinical applications are shown.

Veranstalter
WIAS Berlin
Mittwoch, 07.06.2017, 18:15 Uhr (WIAS-ESH)
Berliner Kolloquium Wahrscheinlichkeitstheorie/Berlin Young Probabilists' Seminar
Prof. Dr. Stefan Adams, University of Warwick, GB:
Large deviations and concentration of scaling limits for weakly pinned integrated random walks
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
We study scaling limits and corresponding large deviation principles of integrated random walks perturbed by an attractive force towards the origin. In particular we analyse the critical situation that the rate function admits more than one minimiser leading to concentration of measure problems. The integrated random walk models are in fact interface models with Laplacian interaction, and such linear chain models with Laplacian interaction appear naturally in the physical literature in the context of semi-flexible polymers. We discuss these connections as well as the ones with the related gradient models. These random fields are a class of model systems arising in the studies of random interfaces, critical phenomena, random geometry, field theory, and elasticity theory. If time permits we outline basic techniques and give an overview about future research projects in this area.

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Berliner Kolloquium Wahrscheinlichkeitstheorie

Veranstalter
Humboldt-Universität zu Berlin
Technische Universität Berlin
Universität Potsdam
WIAS Berlin
Mittwoch, 07.06.2017, 15:15 Uhr (WIAS-406)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Thomas Frenzel, WIAS Berlin:
EDP-convergence for a wiggly energy model
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

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Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
Dienstag, 06.06.2017, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Martin Eigel, WIAS Berlin:
Sampling-free solution of stochastic forward and inverse problems
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
We consider two typical problem classes of UQ: PDEs with random data and Bayesian inversion for these random models. Opposite to classical stochastic approaches, spectral (functional) methods allow for a sampling-free explicit evaluation of probability densities. The formulation is amenable to modern model reduction techniques such as hierarchical tensor approximations and allows for an adaptive error control.

Veranstalter
WIAS Berlin
Donnerstag, 01.06.2017, 16:00 Uhr (WIAS-406)
Forschungsseminar Mathematische Modelle der Photonik
Franziska Bierbüsse, Humboldt-Universität zu Berlin:
Ultrashort dark solitons in the context of the short pulse equation
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Veranstalter
WIAS Berlin
Donnerstag, 01.06.2017, 14:00 Uhr (WIAS-405-406)
Seminar Numerische Mathematik
Dr. Sashikumaar Ganesan, Indian Institute of Science, Bangalore:
Stabilized three-field formulation of viscoelastic fluid flows
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Abstrakt
Viscoelastic flows can be found in a wide range of industrial and commercial applications such as enhanced oil recovery, pesticide deposition, medicinal/pharmaceutical sprays, drug delivery, injection molding, polymer melts, inkjet printing, additive manufacturing, cosmetics industry and food processing. Computations of viscoelastic fluid flows involve simultaneous solutions of the conformation stress tensor equation together with the momentum and mass balance equations. The constitutive equation is advection dominant at high Weissenberg numbers, which necessitates the use of stabilized numerical schemes to avoid local and global oscillations in the numerical solution. The common stabilization schemes used in the context of finite element computations of viscoelastic fluid flows in the literature are Streamline-upwind/Petrov--Galerkin (SUPG), discontinuous Galerkin (DG), discrete Elastic Viscous Stress Splitting (DEVSS) [1], Galerkin Least Square (GLS) [2] and Variational Multiscale method (VMS) [3].

In this talk, a three-field formulation based on the one-level Local Projection Stabilization (LPS) [4, 5] will be presented for computations of Oldroyd-B viscoelastic fluid flows with high Weissenberg numbers. One-level LPS is based on the enriched approximation space and the discontinuous projection space, where both spaces are defined on the same mesh. It allows us to use equal order interpolation spaces for the velocity and viscoelastic stress, whereas inf-sup stable finite elements are used for the velocity and pressure. Further, the coupled system of equations are solved in a monolithic approach. Since LPS stabilization terms are assembled only once, the proposed numerical scheme is computationally efficient in comparison with residual based methods. Numerical studies that show optimal order of convergence in the respective norms will be presented. Further, the numerical results of two benchmark problems: flow past a cylinder in a wide channel and lid-driven cavity flow will be given. Moreover, the effects of elasticity and inertia on the flow dynamics will be discussed.

References:
[1] R. Guenette, A. Fortin, A new mixed finite element method for computing viscoelastic flows, J. Non-Newtonian Fluid Mech., 60 (1995), 27--52.
[2] O. M. Coronado, D. Arora, M. Behr, M. Pasquali, Four-field Galerkin/least-squares formulation for viscoelastic fluids, J. Non-Newtonian Fluid Mech., 140 (2006), 132--144.
[3] E. Castillo, R. Codina, Variational multi-scale stabilized formulation for the stationary three-field incompressible viscoelastic flow problem, Comput. Methods Appl. Mech. Engrg., 279 (2014), 579--605.
[4] G. Matthies, P. Skrzypacz, L. Tobiska, A unified convergence analysis for local projection stabilisations applied to the oseen problem, M2AN Math. Model. Numer. Anal., 41 (2007), 713--742.
[5] S. Ganesan, G. Matthies, L. Tobiska, Local projection stabilization of equal order interpolation applied to the Stokes problem, Math. Comp., 77 (2008), 2039--2060.

Veranstalter
WIAS Berlin
Mittwoch, 31.05.2017, 13:15 Uhr (WIAS-HVP-4.13)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Jo Andrea Brueggemann, Humboldt-Universität zu Berlin:
Solution methods for a contact model motivated by the human heart's pericardium
mehr ... Veranstaltungsort
Weierstraß-Institut, Hausvogteiplatz 11A, 10117 Berlin, 4. Etage, Raum: 4.13

Abstrakt
Introducing a function space description of the contact within the human heart's pericardium, in this talk, we will study a relaxation of the latter. The quasi-variational inequality (QVI) in the focus of interest is attacked with a fixed-point approach and the hereby arising sequence of variational problems can be efficiently solved with a path-following semi-smooth Newton method.

Veranstalter
WIAS Berlin
Mittwoch, 31.05.2017, 10:00 Uhr (WIAS-406)
Forschungsseminar Mathematische Statistik
Prof. Juan Carlos Escanciano, Indiana University Bloomington, USA:
Quantile-regression inference with adaptive control of size
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
Regression quantiles have asymptotic variances that depend on the conditional densities of the response variable given regressors. This paper develops a new estimate of the asymptotic variance of regression quantiles that leads any resulting Wald-type test or con dence region to behave as well in large samples as its infeasible counterpart in which the true conditional response densities are embedded. We give explicit guidance on implementing the new variance estimator to control adaptively the size of any resulting Wald-type test. Monte Carlo evidence indicates the potential of our approach to deliver con dence intervals for quantile regression parameters with excellent coverage accuracy over di erent quantile levels, data-generating processes and sample sizes. We also include an empirical application. Supplementary material is available online.

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Forschungsseminar “Mathematische Statistik”

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Dienstag, 30.05.2017, 15:15 Uhr (WIAS-HVP-3.13)
Oberseminar Nonlinear Dynamics
Phillipo Lappicy, Free University Berlin:
Einstein constraints: A dynamical approach
mehr ... Veranstaltungsort
Weierstraß-Institut, Hausvogteiplatz 11A, 10117 Berlin, 3. Etage, Raum: 3.13

Abstrakt
The Einstein constraint equations describe the space of initial data for the evolution equations, dictating how space should curve within spacetime. Under certain assumptions, the constraints reduce to a scalar quasilinear parabolic equation on the sphere, and nonlinearity being the prescribed scalar curvature of space. We focus on self-similar solutions of Schwarzschild type, which describe the space of initial data of certain black holes, for example. The first main result gives a detailed study of the axially symmetric solutions, since the domain is now one dimensional and nodal properties can be used to describe certain asymptotics of the rescaled self-similar solutions. Such asymptotics describe the possible metrics arising at an event horizon of a black hole, depending on the metric inside the horizon. Those are described by Sturm attractors. In particular, we compute an example for a prescribed scalar curvature. The second main result states a symmetrization property of certain metrics in the event horizon, namely, how the symmetry of the spherical domain can influence the symmetry of solutions.

Veranstalter
Freie Universität Berlin
WIAS Berlin
Dienstag, 30.05.2017, 15:00 Uhr (WIAS-ESH)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Paul Fischer, Humboldt-Universität zu Berlin:
Overview of the paper "Estimating the cluster tree of a density by analyzing the minimal spanning tree of a sample" by Werner Stuetzle
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Dienstag, 30.05.2017, 14:00 Uhr (WIAS-HVP)
Seminar Materialmodellierung
Prof. Ciro Visone, Università del Sannio - Benevento, Italien:
The applicative challenges of Smart Materials: From Sensing to Harvesting
mehr ... Veranstaltungsort
Weierstraß-Institut, Hausvogteiplatz 11A, 10117 Berlin, 4. Etage, Raum: 4.01

Abstrakt
The talk would provide a view on functional materials observed and employed at the macro-scale. Starting from the most known Multi-Functional materials, a common modeling approach, based on the definition of constitutive relationships, is discussed. Examples on the effectiveness of the constitutive equations in the analysis and design problems in engineering is illustrated, along with the basic challenges arising when these materials are of concern. Further, the statement of consistent constitutive relationships when rate-independent memory processes (hysteresis) are considered is also carried out, through the definition of models for multi-input/multi-output systems that formally satisfy the Duhem inequality. Practical examples and specific applications are also proposed.

Veranstalter
WIAS Berlin
Dienstag, 30.05.2017, 13:30 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Prof. Carsten Gräser, Freie Universität Berlin:
Solving nonsmooth PDEs in Dune
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
The design of efficient numerical software for the solution of PDEs is a challenging task because different applications may have very different requirements. As a consequence a large set of libraries has been developed, each with its own focus but in general not compatible with each other.
The central idea of the Dune (distributed unified numerics environment) framework is to change the perspective and to design interfaces that allow to represent a large range of different implementations. This idea is applied on many scales leading to a broad spectrum of interfaces ranging from low level grid operations to high level simulation and code generation frameworks. Based on these principle Dune has evolved to a modular and flexible framework with constantly growing user base and a developer group spread over many institutions.
The talk consists of two parts: The first part will present the Dune framework while the second part briefly outlines how this is used to solve nonsmooth PDEs originating e.g. from applications in material science and biomechanics with state of the art multigrid methods.

Veranstalter
WIAS Berlin
29. Mai – 2. Juni 2017 (WIAS-ESH)
Workshop/Konferenz: CRC 1114 Spring School: Methods for Particle Systems with Multiple Scales
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
SFB 1114: Scaling Cascades in Complex Systems
Mittwoch, 24.05.2017, 18:15 Uhr (WIAS-ESH)
Berliner Kolloquium Wahrscheinlichkeitstheorie/Berlin Young Probabilists' Seminar
Prof. Dr. Christof Külske, Ruhr-Universität Bochum:
Continuous spin models on annealed random graphs: Modifying the modified mean-field exponents
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
We study Gibbs distributions of continuous spins on generalized random graphs. Our main interest lies in the critical behavior of models which show a second order phase transition. We find critical exponents which differ from the mean field exponents when the weight distribution of the generalized random graph becomes too heavy-tailed. For the Ising model this has been proved to occur by Dommers, van der Hofstad, Giardina, and others very recently (CMP 2016). For continuous spins we show in the present analysis that the same modified mean-field exponents known from the Ising model, but also even new universality classes of modified exponents may appear. Joint work with Sander Dommers and Philipp Schriever

Veranstalter
Humboldt-Universität zu Berlin
Technische Universität Berlin
Universität Potsdam
WIAS Berlin
Mittwoch, 24.05.2017, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. D. R. Michiel Renger, WIAS Berlin:
Functions of bounded variation with Banach codomains
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

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Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Mittwoch, 24.05.2017, 13:15 Uhr (WIAS-ESH)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Hongpeng Sun, Renmin University of China, Institute for Mathematical Sciences, China, Volksrepublik:
Weak convergence of proximal ADMM and its relaxations in Hilbert spaces
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
ADMM (Alternating Direction Method of Multipliers) is a popular first order method for mathematical imaging and inverse problems. However, the weak convergence of ADMM in infinite dimensional spaces is not clear yet, which is different from the classical Augmented Lagrangian Method. We will discuss the weak convergence of ADMM and its proximal variants with relaxations in Hilbert spaces.

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Joint Research Seminar on Mathematical Optimization / Non-smooth Variational Problems and Operator Equations

Veranstalter
WIAS Berlin
Mittwoch, 24.05.2017, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Claudia Kirch, Universität Magdeburg:
Frequency domain likelihood approximations for time series bootstrapping and bayesian nonparametrics
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
A large class of time series methods are based on a Fourier analysis, which can be considered as a whitening of the data, giving rise for example to the famous Whittle likelihood. In particular, frequency domain bootstrap methods have been successfully applied in a large range of situations. In this talk, we will rst review existing frequency domain bootstrap methodology for stationary time series before generalizing them for locally stationary time series. To this end, we rst introduce a moving Fourier transformation that captures the time-varying spectral density in a similar manner as the classical Fourier transform does for stationary time series. We obtain consistent estimators for the local spectral densities and show that the corresponding bootstrap time series correctly mimics the covariance behavior of the original time series. The approach is illustrated by means of some simulations and an application to a wind data set. All time series bootstrap methods are implicitely using a likelihood approximation, which could be used explicitely in a Bayesian nonparametric framework for time series. So far, only the Whittle likelihood has been used in this context to get a nonparametric Bayesian estimation of the spectral density of stationary time series. In a second part of this talk we generalize this approach based on the implicit likelihood from the autoregressive aided periodogram bootstrap introduced by Kreiss and Paparoditis (2003). This likelihood combines a parametric approximation with a nonparametric correction making it particularly attractive for Bayesian applications. Some theoretic results about this likelihood approximation including posterior consistency in the Gaussian case are given. The performance is illustrated in simulations and an application to LIGO gravitational wave data.

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Dienstag, 23.05.2017, 15:15 Uhr (WIAS-ESH)
Oberseminar Nonlinear Dynamics
Dr. Pavel Gurevich, Freie Universität Berlin:
A short introduction to machine learning: Towards (un)certainty quantification
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Machine learning is a rapidly developing field that deals with searching for and generating patterns in data. It is nowadays a very broad field encompassing many tasks and methods. In my talk, I will give a short overview, rapidly narrowing my focus towards a particular topic, namely, neural networks and their ability not only to find patterns but also quantify how certain they are in doing so. If time permits, I will conclude with a discussion of our joint work with Hannes Stuke on certainty quantification.

Veranstalter
WIAS Berlin
Freie Universität Berlin
Dienstag, 23.05.2017, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Prof. Archil Gulisashvili, Ohio University, USA:
Extreme-strike asymptotics for general Gaussian stochastic volatility
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Veranstalter
WIAS Berlin
Dienstag, 23.05.2017, 11:00 Uhr (WIAS-406)
Seminar Nichtlineare Optimierung und Inverse Probleme
Robert Gruhlke, WIAS Berlin:
Analysis and Numerics of a Hardy problem featuring an inverse square point potential
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Veranstalter
WIAS Berlin
23. Mai 2017 (WIAS-HVP-3.13)
Workshop/Konferenz: Leibniz MMS Mini Workshop on Mathematical Aspects in Linguistics
mehr ... Veranstaltungsort
Weierstraß-Institut, Hausvogteiplatz 11A, 10117 Berlin, 3. Etage, Raum: 3.13

Veranstalter
WIAS Berlin
Montag, 22.05.2017, 14:00 Uhr (WIAS-ESH)
Seminar Quantitative Biomedizin
Dr. Ralf Blossey, University of Lille 1 & CNRS, Frankreich:
Beyond Poisson-Boltzmann: Charge correlation effects in DNA adsorption and transport through nanopores
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
In soft matter systems electrostatic effects are often of utmost importance. Their mathematical description is commonly based on mean-field type theories, the standard approach being the Poisson-Boltzmann equation. This equation is known to fail if charge correlation effects, e.g., due to the presence of image charges at interfaces, become important. In this talk I will explain how one can go beyond the Poisson-Boltzmann equation in order to treat such mechanisms. As two exemplary applications I will discuss the correlation-induced adsorption of DNA on like-charged surfaces and the conversion of ionic currents in DNA translocation through nanopores.

Veranstalter
WIAS Berlin
18. Mai 2017 – 20. Mai 2016 (WIAS-ESH)
Workshop/Konferenz: 7th Annual ERC Berlin-Oxford Young Researchers Meeting on Applied Stochastic Analysis
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
European Research Council
18. – 20. Mai 2017 (WIAS-ESH)
Workshop/Konferenz: 7th Annual ERC Berlin-Oxford Young Researchers Meeting on Applied Stochastic Analysis
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
European Research Council
WIAS Berlin
Mittwoch, 17.05.2017, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar) & Seminar Materialmodellierung
Dr. Riccarda Rossi, Università di Brescia, Italien:
In Between Energetic and Balanced Viscosity solutions of rate-independent systems: The Visco-Energetic concept, with some applications to solid mechanics
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
This talk focuses on weak solvability concepts for rate-independent systems. Visco-Energetic solutions have been recently obtained by passing to the time-continuous limit in a time-incremental scheme, akin to that for Energetic solutions, but perturbed by a `viscous' correction term, as in the case of Balanced Viscosity solutions. However, for Visco-Energetic solutions this viscous correction is tuned by a fi xed parameter. The resulting solution notion is characterized by a stability condition and an energy balance analogous to those for Energetic solutions, but, in addition, it provides a fine description of the system behavior at jumps as Balanced Viscosity solutions do. Visco-Energetic evolution can be thus thought as `in-between' Energetic and Balanced Viscosity evolution. We will explore these aspects in a general metric framework. We will then illustrate the application of the Visco-Energetic concept to models for damage and finite-strain plasticity. Joint work with Giuseppe Savaré.

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Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
Mittwoch, 17.05.2017, 09:30 Uhr (WIAS-406)
Halbleiterseminar
Dr. Sina Reichelt, WIAS:
Pulses in FitzHugh-Nagumo systems with periodic coefficients
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Weitere Informationen
Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Veranstalter
WIAS Berlin
Dienstag, 16.05.2017, 15:15 Uhr (WIAS-ESH)
Oberseminar Nonlinear Dynamics
Hannes Stuke, Freie Universität Berlin:
Parabolic blow-up in complex time
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
In my talk I will give an overview of my research on parabolic differential equations and blow-up. Blow-up denotes the phenomenon, when solutions diverge to infinity. By considering complex time we try to establish a connection between two opposed worlds: on the one hand bounded solutions, in particular equilibria and heteroclinic orbits and on the other hand blow-up in equations with entire nonlinearities.

Veranstalter
WIAS Berlin
Freie Universität Berlin
Donnerstag, 11.05.2017, 16:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Modelle der Photonik
Dr. Mindaugas Radziunas, WIAS Berlin:
A multi-mode delay differential equation model for lasers with optical feedback
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Mittwoch, 10.05.2017, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Marion Dziwnik, WIAS Berlin:
The role of degenerate mobilities in Cahn-Hilliard models
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
Mittwoch, 10.05.2017, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Marco Cuturi, École Nationale de la Statistique et de l'Administration Économique, Frankreich:
A review of regularized optimal transport and applications to Wasserstein barycenters
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Mittwoch, 10.05.2017, 09:30 Uhr (WIAS-406)
Halbleiterseminar
Dr. Joachim Rehberg, WIAS Berlin:
Van Roosbroeck's system is well-posed in case of general recombination terms
Part II: The analysis of the system
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Weitere Informationen
Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Veranstalter
WIAS Berlin
Dienstag, 09.05.2017, 15:30 Uhr (WIAS-HVP-4.13)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Professor Kazufumi Ito, North Carolina State University, USA:
Value function calculus and Applications
mehr ... Veranstaltungsort
Weierstraß-Institut, Hausvogteiplatz 11A, 10117 Berlin, 4. Etage, Raum: 4.13

Abstrakt
In this talk the sensitivity analysis is discussed for the parameter-dependent optimization. The sensitivity of the optimal value function with respect to the change in parameters plays a significant role in the optimization theory, including economics, finance, the Hamilton-Jacobi theory, the inf-sup duality and the structural design and the bi-level optimization. We develop the calculus for the value function and present its applications in the variational calculus, the bi-level optimization and the optimal control and optimal design and inverse problems.

Veranstalter
WIAS Berlin
Dienstag, 09.05.2017, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Prof. Andrey Sobolevskiy, Russian Academy of Sciences, IITP, Moscow:
The Hamilton-Jacobi equation: Parallel transport in the 2-Wasserstein space and beyond
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
The Hamilton-Jacobi equation with quadratic Hamiltonian describes parallel transport in the 2-Wasserstein space of measures. While geodesics in Wasserstein spaces arise as solutions to two-point boundary value problem, the natural setting for the Hamilton-Jacobi equation is an initial value problem. We will describe the corresponding dynamics for measures, first constructed by Ilya Bogaevski for the quadratic Hamiltonian, and its extension to a general convex Hamiltonian due to Konstantin Khanin and the speaker.

Veranstalter
WIAS Berlin
Dienstag, 09.05.2017, 14:15 Uhr (WIAS-ESH)
Seminar Nichtlineare Optimierung und Inverse Probleme
Ahmad Ali, Universität Hamburg:
Global minima for semilinear optimal control problems
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Dienstag, 09.05.2017, 14:00 Uhr (WIAS-406)
Seminar Materialmodellierung
Dr. Mathias Schäffner, Technische Universität Dresden:
Stochastic homogenization of discrete energies with degenerate growth
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
We present a discrete-to-continuum analysis for lattice systems with random interactions. In particular, we assume that the interaction potentials satisfy polynomial growth conditions which degenerate and are given in terms of certain weight functions. Under suitable moment conditions on the weight functions and stationarity/ergodicity assumptions for the interaction potentials, we prove that the discrete energy Gamma-converges almost surely to a deterministic, homogeneous and non-degenerate integral functional. This is joint work with S. Neukamm (TU Dresden) and A. Schlömerkemper (U Würzburg)

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Seminar “Materialmodellierung”

Veranstalter
WIAS Berlin
Dienstag, 09.05.2017, 13:00 Uhr (WIAS-406)
Seminar Materialmodellierung
Dr. Martin Slowik, Technische Universität Berlin:
Random conductance model in a degenerate ergodic environment
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
Consider a continuous time random walk on the Euclidean lattice $mathbbZ^d$ in an environment of random conductances taking values in $[0, infty)$. The law of the environment is assumed to be ergodic with respect to space shifts and satisfies some moment conditions. In this talk, I will review old and discuss recent results on quenched invariance principles (an instance of stochastic homogenization in path space), local limit theorems as well as heat kernel estimates for this Markov process. This is joint work with Sebastian Andres (Univ. Cambridge), Jean-Dominique Deuschel (TU Berlin) and Tuan Ahn Nguyen (TU Berlin).

Veranstalter
WIAS Berlin
Dienstag, 09.05.2017, 10:15 Uhr (WIAS-406)
Seminar Nichtlineare Optimierung und Inverse Probleme
Dr. Oliver Lass, Technische Universität Darmstadt:
Nonlinear robust optimization and model order reduction with application to electric motor design
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Veranstalter
WIAS Berlin
Freitag, 05.05.2017, 14:15 Uhr (WIAS-ESH)
Seminar Nichtglatte Variationsprobleme und Operatorgleichungen
Dr. Behzad Azmi, Universität Graz, Österreich:
On the stabilizability of infinite dimensional systems via receding horizon control
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
One efficient strategy for dealing with optimal control problems on an infinite time horizon is the receding horizon framework. In this approach, an infinite horizon optimal control problem is approximated by a sequence of finite horizon problems in a receding horizon fashion. Stability is not generally ensured due to the use of a finite prediction horizon. Thus, in order to ensure the asymptotic stability of the controlled system, additional terminal cost functions and/or terminal constraints are often needed to add to the finite horizon problems. In this presentation, we are concerned with the stabilization of several classes of infinite-dimensional controlled systems by means of a Receding Horizon Control (RHC) scheme. In this scheme, no terminal costs or terminal constraints are used to ensured the stability. The key assumption is the stabilizability of the underlying system. Based on this condition the suboptimality and stability of RHC are investigated. To justify the applicability of this framework, we consider controlled systems governed by different types of partial differential equations. Numerical examples are presented as well.

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Seminar RG 8

Veranstalter
WIAS Berlin
Donnerstag, 04.05.2017, 16:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Modelle der Photonik
Dr. Dirk Peschka, WIAS Berlin:
Gradient structure for optoelectronic models of semiconductors
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Donnerstag, 04.05.2017, 10:15 Uhr (WIAS-ESH)
Seminar Nichtlineare Optimierung und Inverse Probleme
Prof. Dr. Elisabetta Rocca, Università degli Studi di Pavia, Italien:
Optimal control in diffuse interface models of tumor growth
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Donnerstag, 04.05.2017, 09:30 Uhr (HU Berlin in Adlershof RUD25 Raum 2.417)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Axel Kröner, HU Berlin/CMAP, Ecole Polytechnique, Paris-Saclay:
Optimal control of infinite dimensional systems
mehr ... Veranstaltungsort
Humboldt-Universität zu Berlin, Rudower Chaussee 25, 12489 Berlin, Johann-von-Neumann-Haus, Raum 2.417

Abstrakt
In this talk we analyze second order optimality conditions for a bilinear optimal control problem governed by a strongly continuous semigroup operator, the control entering linearly in the cost function. We derive first and second order optimality conditions, taking advantage of the Goh transform. The general framework allows the application to heat, wave, and Schrödinger equation.

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Mittwoch, 03.05.2017, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Martin Heida, WIAS Berlin:
Convergence of the square-root approximation scheme to the Fokker--Planck equation
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

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Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
Mittwoch, 03.05.2017, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Cristina Butucea, Université Paris-Est Marne-la-Vallée, Frankreich:
Local asymptotic equivalence for quantum models
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Quantum statistics is concerned with inference for physical systems described by quantum mechanics. After an introduction to the main notions of quantum statistics: quantum states, measure- ments, channels, we describe nonparametric quantum models. We prove the local asymptotic equivalence (LAE) of i.i.d. quantum pure states and a quantum Gaussian state, in the sense of Le Cam theory. As an application, we show the optimal rates for the estimation of pure states, for the estimation of some quadratic functionals and for the testing of pure states. Surprisingly, a sharp parametric testing rate is obtained in a nonparametric quantum setup. Joint work with M. Guta and M. Nussbaum.

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Forschungsseminar “Mathematische Statistik”

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Mittwoch, 03.05.2017, 09:30 Uhr (WIAS-HVP-3.13)
Halbleiterseminar
Dr. Joachim Rehberg, WIAS Berlin:
Van Roosbroeck's system is well-posed in case of general recombination terms
Part I: Functional analytic preliminaries
mehr ... Veranstaltungsort
Weierstraß-Institut, Hausvogteiplatz 11A, 10117 Berlin, 3. Etage, Raum: 3.13

Veranstalter
WIAS Berlin
Dienstag, 02.05.2017, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Aleksei Kroshnin, MIPT, Russian Academy of Sciences , Russische Föderation:
Fréchet Barycenters in the Monge-Kantorovich spaces
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
Let P(X) be a space of probability measures on an arbitrary Polish space X. We will discuss a Mongeâ??Kantorovich distance with an arbitrary cost function, given as a solution of optimal transportation problem between two measures from P(X). A useful property of this distance is that it takes into account the geometry of underlying space X. We will show that for a wide class of cost functions the Mongeâ??Kantorovich distance generates a topology on P(X) and consider the properties of this space. Furthermore, we will introduce the notion of Fréchet barycenter which is a generalization of Wasserstein barycenters. It will be shown that the Fréchet barycenters have some regular properties, e.g. they are continuous, consistent etc.

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Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics

Veranstalter
WIAS Berlin
Donnerstag, 27.04.2017, 10:00 Uhr (WIAS-ESH)
Seminar Laserdynamik
Sindre W. Haugland, Technische Universität München:
What can we learn about chimera states from minimal cluster dynamics?
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

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Seminar Laserdynamik

Veranstalter
WIAS Berlin
Mittwoch, 26.04.2017, 18:15 Uhr (WIAS-ESH)
Berliner Kolloquium Wahrscheinlichkeitstheorie/Berlin Young Probabilists' Seminar
Prof. Dr. Andrey Pilipenko, Ukrainian National Academy of Sciences, Ukraine:
On a selection problem for small noise perturbation of ODE in multidimensional case
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
The identification problem of the limit of an ODE with non-Lipschitz drift perturbed by a zero-noise is considered in a multidimensional framework. This problem is a classical subject of stochastic analysis, however the multidimensional case was poorly investigated. We consider two cases in particular:
(i) the drift coefficient has a jump discontinuity along an hyperplane and is Lipschitz continuous in the upper and lower half-spaces;
(ii) the drift is equivalent to a(phi) $r^alpha$ as $r$ tends to $0$, where (r,phi) are the polar coordinates, and alpha$<1$.

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Berliner Kolloquium Wahrscheinlichkeitstheorie

Veranstalter
Humboldt-Universität zu Berlin
Technische Universität Berlin
Universität Potsdam
WIAS Berlin
Mittwoch, 26.04.2017, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Dirk Peschka, WIAS Berlin:
Motion of thin droplets over surfaces - Numerical and modeling techniques for moving contact lines
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Weitere Informationen
Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
Mittwoch, 26.04.2017, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Jonathan Weed, Massachusetts Institute of Technology, USA:
Optimal rates of estimation for the multi-reference alignment problem
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
How should one estimate a signal, given only access to noisy versions of the signal corrupted by unknown circular shifts? This simple problem has surprisingly broad applications, in fields from structural biology to aircraft radar imaging. We describe how this model can be viewed as a multivariate Gaussian mixture model whose centers belong to an orbit of a group of orthogonal transformations. This enables us to derive matching lower and upper bounds for the optimal rate of statistical estimation for the underlying signal.. These bounds show a striking dependence on the signal-to-noise ratio of the problem. Joint work with Afonso Bandeira and Philippe Rigollet.

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Forschungsseminar “Mathematische Statistik”

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
International Research Training Group 1792
Dienstag, 25.04.2017, 15:15 Uhr (WIAS-ESH)
Oberseminar Nonlinear Dynamics
Juliana Pimentel, Universidade Federal do ABC:
Permutation characterization for slowly non-dissipative equations
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
We study the longtime behavior of slowly non-dissipative scalar reaction-diffusion equations. By extending known results, we are able to obtain a complete decomposition of the non-compact global attractor and still manage to determine the heteroclinic connections based on the Sturm permutation method.

Veranstalter
Freie Universität Berlin
WIAS Berlin
Dienstag, 25.04.2017, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Nikita Zhivotovskiy, IITP RAS, SkolTech:
Towards minimax optimal rates in classification and regression
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
In this talk, we consider two approaches that allow (in some cases) to obtain minimax rates of the prediction risk up to absolute constants. Classification problems will be considered under small noise conditions for an arbitrary distribution of objects, as well as cases of certain special distributions. In contrast to several standard results in the learning theory, our bounds are simultaneously optimal for entire families of classes.

Veranstalter
WIAS Berlin
Dienstag, 25.04.2017, 14:15 Uhr (WIAS-ESH)
Seminar Nichtlineare Optimierung und Inverse Probleme
Prof. Dr. Masahiro Yamamoto, University of Tokyo, Japan:
Inverse problems for an integro-hyperbolic equation for the viscoelasticity
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Dienstag, 25.04.2017, 14:00 Uhr (WIAS-406)
Seminar Materialmodellierung
Ian Thompson, University of Bath, GB:
Modelling device charge dynamics on the microscopic scale
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
We attempt to predict the properties of organic semiconductor (OSC) materials using a microscopic ab initio approach. Charge transport through organic semiconductors (OSCs) is qualitatively different from metallic semiconductors, charges hop between molecules discretely. Marcus theory describes the microscopic hopping mechanism, quantum chemistry methods can calculate the parameters and kinetic Monte Carlo methods can be used to model charge motion. We also need to describe realistic configurations of a set of given molecules. To combine all of these approaches into a single multi-scale model is the goal of the EXTMOS project. We present simulations of charge carrier motion in a system of discotic molecules with high levels of shape anisotropy; using explicitly calculated parameters we are able to capture and quantify the effect on charge transport anisotropy. We also consider the use of network models to describe collective behaviour.

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Seminar “Materialmodellierung”

Veranstalter
WIAS Berlin
Dienstag, 25.04.2017, 10:15 Uhr (WIAS-406)
Seminar Nichtlineare Optimierung und Inverse Probleme
Dr. Jan Schwientek, Fraunhofer-Institut für Techno- und Wirtschaftsmathematik, Kaiserslautern:
Numerical methods for general(ized) semi-infinite optimization -- Applied to gemstone cutting
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
The talk is divided into two parts. In the first part I will give a short introduction to general(ized) semi-infinite programming (GSIP) and survey the numerical methods. At that we will look at one method in more detail, the transformation-based discretization method. It cleverly combines transforming a GSIP into a usual SIP problem and solving the latter one via discretization and exploiting the lower level convexity of the underlying GSIP problem.

The second part of my talk will focus on the most shimmering application of general semi-infinite optimization, the volume-maximal utilization of (colored) gemstones. There, the task is the following: Given an irregular raw gem which is provided with surface defects and interspersed with inclusions, produce a set of precious gems such that the profit is as high as possible. I will show how this task can be modeled and solved as a general semi-infinite optimization problem, and how we installed a new fully automatic industrial production process for producing the jewels.

Veranstalter
WIAS Berlin
Mittwoch, 19.04.2017, 18:15 Uhr (WIAS-ESH)

Prof. Dr. Frank den Hollander, Leiden University, Niederlande:
Random walks on dynamic random graphs
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
The mixing time of a Markov chain is the time it needs to approach its stationary distribution. For random walks on graphs, the characterisation of the mixing time has been the subject of intensive study. One of the motivations is the fact that the mixing time gives information about the geometry of the graph. In the last few years, much attention has been devoted to the analysis of mixing times for random walks on emphrandom graphs, which poses interesting challenges. Many real-world networks are dynamic in nature. It is therefore natural to study random walks on emphdynamic random graphs. In this talk we consider a random walk on the configuration model, i.e., a random graph with prescribed degrees. We investigate what happens when at each unit of time a fraction $alpha_n$ of the edges is randomly relocated, where $n$ is the number of nodes. We identify emphthree regimes for the mixing time in the limit as $n to infty$, depending on the choice of $alpha_n$. These regimes exhibit surprising behaviour. Joint work with Luca Avena (Leiden), Hakan Guldas (Leiden) and Remco van der Hofstad (Eindhoven)

Veranstalter
Humboldt-Universität zu Berlin
Technische Universität Berlin
Universität Potsdam
WIAS Berlin
Mittwoch, 19.04.2017, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Dr. Christian Kühn, Technische Universität München:
Regularity structures and fractional diffusion
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

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Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
Dienstag, 18.04.2017, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Yangwen Sun, HU Berlin:
Minimum spanning tree approach for change-point detection
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
Change-point detection has been demonstrated to be useful in various areas. As the dimension and size of the data increase, the detection of the change-point becomes more challenging. A recent graph-based approach by Chen and Zhang (2015) has claimed to be effective especially in the high dimensional case. In this talk, we will focus on an approach based on minimum spanning tree (MST) graph. The change-point detection problem is characterized as a test of a significant change in the probability distribution. The proposed MST-based dissimilarity measure, the selection of critical level for the test, and the performance of the approach will be presented in this talk.

Veranstalter
WIAS Berlin
Dienstag, 11.04.2017, 15:15 Uhr (WIAS-ESH)
Seminar Laserdynamik
Prof. Dmitry Turaev, Imperial College London, GB:
Energy equilibration in slow-fast systems
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
We consider a slow-fast Hamiltonian system where the evolution of some degrees of freedom is slower than the evolution of others. It is well-known that the ergodicity of the fast subsystem prevents the full slow-fast system of achieving the state of statistical equilibrium.. We show that the violation of ergodicity in the fast subsystem drives the full system to equilibrium in an effective way.

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Seminar Laserdynamik

Veranstalter
WIAS Berlin
Dienstag, 11.04.2017, 14:00 Uhr (WIAS-406)
Seminar Materialmodellierung
Prof. Luca Heltai, SISSA mathLab, Triest, Italien:
A numerical framework for optimal locomotion at low Reynolds numbers
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
Swimming (advancing in a fluid in the absence of external propulsive forces by performing cyclic shape changes) is particularly demanding at low Reynolds numbers. This is the regime of interest for microorganisms and micro- or nano-robots, where hydrodynamics is governed by Stokes equations, and swimming is complicated by the fact that viscosity dominates over all participating forces. We exploit a formulation of the swimming problem in the context of Control Theory, and we present a numerical approximation scheme based on Boundary Element Methods (BEM) and reduced space Successive Quadratic Programming (rSQP) that is capable of computing efficiently optimal strokes for a variety of micro swimmers, both biological and artificial. We apply this framework to the study of the locomotion of euglenids (one of the best-known groups of flagellates). These organisms exhibit an unconventional motility strategy amongst unicellular eukaryotes, consisting of large-amplitude highly concerted deformations of the entire body (euglenoid movement or metaboly). We identify previously unnoticed features of metaboly, and we find that metaboly accomplishes locomotion at hydrodynamic efficiencies comparable to those of ciliates and flagellates. Our results suggest new quantitative experiments, provide insight into the evolutionary history of euglenids, and suggest that the pellicle may serve as a model for engineered active surfaces with applications in microfluidics.

Veranstalter
WIAS Berlin
Dienstag, 11.04.2017, 13:30 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Prof. Jianjun Chen, Zheijian University, China:
Automatic and parallel mesh generation: Recent advances
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Mesh generation is recognized as the main performance bottleneck in simulations involving complicated geometric definitions and/or complex physical behaviors. Automatic generation of high-quality meshes remains a challenging issue. Besides, quite a few simulations are now demanding meshes consisting in hundreds of millions of elements or more. It could be very difficult and inefficient to create such big meshes by sequential algorithms due to the bottlenecks of memory usage and computing time.
In this talk, Dr. Chen will present three technologies developed by his research group recently, as part of his efforts to develop a completely automatic and parallel meshing system for real-world simulations. These technologies are: (1) Automatic creation of a feature-aware and gradient-limited sizing function; (2) Automatic generation of high-quality boundary layer elements using vector fields computed by the boundary element method; (3) Distributed and/or multi-threaded parallel mesh generation and mesh quality improvement. Mathematic models behind these technologies will be investigated in this talk, plus important algorithmic details. Meshing examples are selected to show that the developed technologies are applicable to configurations of a complication level experienced in industry.

Veranstalter
WIAS Berlin
Donnerstag, 06.04.2017, 14:00 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Prof. David Silvester, University of Manchester, GB:
Accurate time-integration strategies for modelling incompressible flow bifurcations
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Eigenvalue analysis is a well-established tool for stability analysis of dynamical systems. However, there are situations where eigenvalues miss some important features of physical models. For example, in models of incompressible fluid dynamics, there are examples where linear stability analysis predicts stability but transient simulations exhibit significant growth of infinitesimal perturbations. In this study, we show that an approach similar to pseudo-spectral analysis can be performed inexpensively using stochastic collocation methods and the results can be used to provide quantitive information about the nature and probability of instability.

Veranstalter
WIAS Berlin
Dienstag, 04.04.2017, 15:15 Uhr (WIAS-ESH)
Seminar Laserdynamik
Prof. Edgar Knobloch, University of California, USA:
Geostrophic turbulence and the formation of large scale structure
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Low Rossby number convection is studied using an asymptotically reduced system of equations valid in the limit of strong rotation. The equations describe four regimes as the Rayleigh number $Ra$ increases: a disordered cellular regime near threshold, a regime of weakly interacting convective Taylor columns at larger $Ra$, followed for yet larger $Ra$ by a breakdown of the convective Taylor columns into a disordered plume regime characterized by reduced heat transport efficiency, and finally by geostrophic turbulence. The Nusselt number--Rayleigh number scaling in the ültimate" regime of geostrophic turbulence is predicted and confirmed using direct numerical simulations of the reduced equations. These simulations reveal that geostrophic turbulence is unstable to the formation of large scale barotropic vortices, via a process known as spectral condensation. The details of this process are quantified and its implications explored.

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Seminar Laserdynamik

Veranstalter
WIAS Berlin
Donnerstag, 30.03.2017, 14:00 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Prof. J.H.M. ten Thije Boonkkamp, Eindhoven University of Technology, Netherlands:
Complete flux schemes fOR conservation laws of advection-diffusion-reaction typ
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Complete flux schemes are recently developed numerical flux approximation schemes for conservation laws of advection-diffusion-reaction type; see e.g. [1, 2]. The basic complete flux scheme is derived from a local one-dimensional boundary value problem for the entire equation, including the source term. Consequently, the integral representation of the flux contains a homogeneous and an inhomogeneous part, corresponding to the advection-diffusion operator and the source term, respectively. Suitable quadrature rules give the numerical flux. For time-dependent problems, the time derivative is considered a source term and is included in the inhomogeneous flux, resulting in an implicit semi-discretisation. The implicit system proves to have much smaller dissipation and dispersion errors than the standard semidiscrete system, especially for dominant advection. Just as for scalar equations, for coupled systems of conservation laws, the complete flux approximation is derived from a local system boundary value problem, this way incorporating the coupling between the constituent equations in the discretization. Also in the system case, the numerical flux (vector) is the superpostion of a homogeneous and an inhomogeneous component, corresponding to the advection-diffusion operator and the source term vector, respectively. The scheme is applied to multispecies diffusion and satisfies the mass constraint exactly.
References:
[1] J.H.M. ten Thije Boonkkamp and M.J.H. Anthonissen, “The finite volume-complete flux scheme for advection-diffusion-reaction equations”, J. Sci. Comput., 46, 47--70, (2011).
[2] J.H.M. ten Thije Boonkkamp, J. van Dijk, L. Liu and K.S.C. Peerenboom, “Extension of the complete flux scheme to systems of comservation laws”, J. Sci. Comput., 53, 552?568, (2012).

Veranstalter
WIAS Berlin
Mittwoch, 29.03.2017, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Dr. Michael Demuth, Technische Universität Clausthal:
On eigenvalues of non-selfadjoint operators: A comparison of two approaches
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

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Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
Mittwoch, 29.03.2017, 13:15 Uhr (WIAS-ESH)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Prof. Christian Clason, Universität Duisburg-Essen:
Convex relaxation of hybrid discrete-continuous control problems
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Dienstag, 28.03.2017, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Evgeny Burnaev, Skolkovo Institute of Science and Technology , Russische Föderation:
Minimax approach to variable fidelity data interpolation
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
In this work we obtain minimax interpolation errors for single and variable fidelity scenarios for a multivariate Gaussian process regression. Evaluation of the minimax errors allows us to identify cases when the variable fidelity data provides better interpolation accuracy than the exclusively high fidelity data for the same computational budget. These results allow us to calculate the optimal shares of variable fidelity data samples under the given computational budget constraint. Real and synthetic data experiments suggest that using the obtained optimal shares often outperforms natural heuristics in terms of the regression accuracy.
References:

Gaussian Models for Data Fusion - Minimax Error of Interpolation and Optimal Design of Experiments for Variable Fidelity Data (https://goo.gl/CjgLBE) - Surrogate modeling of multifidelity data for large samples (https://goo.gl/QFkrxf)

Gaussian Process Regression and its Properties - Regression on the Basis of Nonstationary Gaussian Processes with Bayesian Regularization (https://goo.gl/haZsfM) - The Bernstein-von Mises theorem for regression based on Gaussian Processes (https://goo.gl/9whF3z) - Properties of the bayesian parameter estimation of a regression based on gaussian processes (https://goo.gl/dMd4mD)

Efficient Learning of Gaussian Process regression - Computationally Efficient Algorithm for Gaussian Process Regression in Case of Structured Samples (https://goo.gl/uXzDfW) - Adaptive Design of Experiments Based on Gaussian Processes (https://goo.gl/EjE9kt)

Surrogate Modeling for Industrial Design - GTApprox: Surrogate modeling for industrial design (https://goo.gl/mz7QtV)
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Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics

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WIAS Berlin

Dienstag, 28.03.2017, 14:30 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Prof. Luca Heltai, SISSA, Italien:
Immersed Finite Element Methods for interface and fluid structure interaction problems: An overview and some recent results
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Immersed Finite Element Methods (IFEM) are an evolution of the original Immersed Boundary Element Method (IBM) developed by Peskin in the early seventies for the simulation of complex Fluid Structure Interaction (FSI) problems. In the IBM, the coupled FSI problem is discretised using a single (uniformly discretised) background fluid solver, where the presence of the solid is taken into account by adding appropriate forcing terms in the fluid equation. Approximated Dirac delta distributions are used to interpolate between the Lagrangian and the Eulerian framework in the original formulation by Peskin, while a variational formulation was introduced by Boffi and Gastaldi (2003), and later generalised by Heltai and Costanzo (2012). By carefully exploiting the variational definition of the Dirac distribution, it is possible to reformulate the discrete Finite Element problem using non-matching discretisations without recurring to Dirac delta approximation.
One of the key issues that kept people from adopting IBM or IFEM techniques is related to the loss in accuracy attributed to the non-matching nature of the discretisation between the fluid and the solid domains, leading to solvers that converge only sub-optimally.
In this talk I will present a brief overview of Immersed Finite Element Methods, and will present some recent results that exploit techniques introduced by D?Angelo and Quarteroni (2012), to show that, for the variational finite element formulation, the loss in accuracy is only restricted to a thin layer of elements around the solid-fluid interface, and that optimal error estimates in all norms are recovered if one uses appropriate weighted norms when measuring the error.

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WIAS Berlin
Dienstag, 28.03.2017, 13:30 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Giuseppe Pitton, SISSA, Italien:
Accelerating augmented and deflated Krylov space methods for convection-diffusion problems
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
In this talk I will recall some basic notions of augmented and deflated Krylov space methods for the iterative solution of linear systems. Then I will discuss a few strategies to apply these techniques to the solution of linear systems coming from nonlinear convection-diffusion equations . In particular, I will argue that in some cases it may be convenient to exploit some alternative recycling strategies based on the SVD selection of previous solutions. Some numerical tests in scalar nonlinear convection-diffusion problems discretized with Finite Elements and Spectral Elements will be discussed.

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WIAS Berlin
22. – 24. März 2017 (WIAS-ESH)
Workshop/Konferenz: Nonlinear Waves and Turbulences in Optics and Hydrodynamics
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

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WIAS Berlin
Dienstag, 21.03.2017, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Andzhey Koziuk, WIAS Berlin:
Some ideas on Gaussian comparison and approximation
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
In the talk I plan to discuss new ideas and bottlenecks in the current state of research on how to upper bound Kolmogorov distance on Euclidean balls. The talk will be largely continuation of the one I gave in December 2016 and will pursue the same questions:
1. What is optimal dimensional dependence (both for comparison and approximation) of the upper-bound
2. Ways to improve Gaussian approximation

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WIAS Berlin
Montag, 20.03.2017, 14:00 Uhr (WIAS-ESH)
Seminar Quantitative Biomedizin
Thoralf Niendorf, Charité Max Delbrück Center for Molecular Medicine:
Explorations into ultrahigh field magnetic resonance
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
The development of ultrahigh field magnetic resonance (UHF-MR) is moving forward at an amazing speed that is breaking through technical barriers almost as fast as they appear. UHF-MR has a staggering number of potential uses in neuroscience, neurology, radiology, cardiology, internal medicine, physiology, oncology, nephrology, ophthalmology and other related clinical fields. With almost 40,000 MR examinations already performed at 7.0 Tesla, the reasons for moving UHF-MR into clinical applications are more compelling than ever.

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WIAS Berlin
Donnerstag, 16.03.2017, 14:00 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Mattéo Cicuttin, CERMICS - ENPC, Frankreich:
Implementation of Discontinuous Skeletal methods on arbitrary-dimensional, polytopal meshes using generic programming
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Discontinuous Skeletal methods are devised at the mathematical level in a dimension-independent and cell-shape-independent fashion. Their implementation, at least in principle, should conserve this feature: a single piece of code should be able to work in any space dimension and to deal with any cell shape. It is not common, however, to see software packages taking this approach. In the vast majority of the cases, the codes are capable to run only on few very specific kinds of mesh, or only in 1D or 2D or 3D. On the one hand, this can happen simply because a fully general tool is not always needed. On the other hand, the programming languages commonly used by the scientific computing community (in particular Fortran and Matlab) are not easily amenable to an implementation which is generic and efficient at the same time. The usual (and natural) approach, in conventional languages, is to have different versions of the code, for example one specialized for 1D, one for 2D and one for 3D applications, making the overall maintenance of the codes rather cumbersome. The same considerations generally apply to the handling of mesh cells with various shapes, i.e., codes written in conventional languages generally support only a limited (and set in advance) number of cell shapes.
Generic programming offers a valuable tool to address the above issues: by writing the code generically, it is possible to avoid making any assumption neither on the dimension (1D, 2D, 3D) of the problem, nor on the kind of mesh. In some sense, writing generic code resembles writing pseudocode: the compiler will take care of giving the correct meaning to each basic operation. As a result, with generic programming there will be still differents versions of the code, but they will be generated by the compiler, and not by the programmer. As these considerations suggest, generic programming is a static technique: if correctly realized, the abstractions do not penalize the performance at runtime, because they will leave no trace in the generated code.
In this talk we will discuss how the Hybrid High Order method is implemented atop of DiSk++, a newly developed library for the generic implementation of Discontinuous Skeletal methods.

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WIAS Berlin
Dienstag, 14.03.2017, 10:15 Uhr (WIAS-406)
Seminar Nichtlineare Optimierung und Inverse Probleme
Dr. Joachim Linn, Fraunhofer-Institut für Techno- und Wirtschaftsmathematik, Kaiserslautern:
Simulation of flexible cables in car assembly
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

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WIAS Berlin
Freitag, 03.03.2017, 10:00 Uhr (WIAS-ESH)
Seminar Partielle Differentialgleichungen
Stefano Melchionna, University of Vienna, Österreich:
A variational approach to symmetry, monotonicity and comparison for doubly-nonlinear equations
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
We advance a variational method to prove qualitative properties such as symmetries, monotonicity, upper and lower bounds, sign properties, and comparison principles for a large class of doubly-nonlinear evolutionary problems including gradient flows, some nonlocal problems, and systems of nonlinear parabolic equations. Our method is based on the so-called Weighted-Energy-Dissipation (WED) variational approach. This consists in defining a global parameter-dependent functional over entire trajectories and proving that its minimizers converge to solutions to the target problem as the parameter goes to zero. Qualitative properties and comparison principles can be easily proved for minimizers of the WED functional and, by passing to the limit, for the limiting problem. Several applications of the abstract results to systems of nonlinear PDEs and to fractional/nonlocal problems are presented.

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Seminar Partielle Differentialgleichungen

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WIAS Berlin
Donnerstag, 02.03.2017, 14:00 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Dr. Lucas Omar Müller, Norwegian University of Science and Technology:
A local time stepping solver for one-dimensional blood flow
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
We present a finite volume solver for one-dimensional blood flow simulations in networks of elastic and viscoelastic vessels, featuring high-order space-time accuracy and local time stepping (LTS). The solver is built on:
(i) a high-order finite-volume type numerical scheme,
(ii) a high-order treatment of the numerical solution at internal vertexes of the network (junctions);
(iii) an accurate LTS strategy.
Several applications of the method will be presented. First, we apply the LTS scheme to the Anatomically Detailed Arterial Network model (ADAN), comprising 2142 arterial vessels. Second, we show results of a computational study where the ADAN model is coupled to automatically generated microvascular networks in order to elucidate aspects on the pathopysiology of small vessel disease for cerebral arteries.

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WIAS Berlin
Donnerstag, 02.03.2017, 09:00 Uhr (WIAS-406)
Halbleiterseminar
Dr. Duy Hai Doan, WIAS:
A unified Scharfetter Gummel scheme
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

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Halbleiterseminar

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WIAS Berlin
Mittwoch, 01.03.2017, 15:15 Uhr (WIAS-406)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Dr. Pierluigi Colli, University of Pavia, Italien:
About a non-smooth regularization of a forward-backward parabolic equation
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
A variation of the Cahn-Hilliard equation, describing diffusion of species by a suitable regularization of a "forward-backward" parabolic equation is discussed. In the concerned system, the general viscous regularization is expressed by a maximal monotone graph acting on the time derivative of the concentration and presenting a strong coerciveness property. The phase variable stands for the concentration of a chemical species and it evolves under the influence of a non-convex free energy density. For the chemical potential a non-homogeneous Dirichlet boundary condition is assumed. Existence and continuous dependence results are shown. The talk reports on a joint work with E. Bonetti and G. Tomassetti.

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Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

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WIAS Berlin
Humboldt-Universität zu Berlin
Donnerstag, 23.02.2017, 09:30 Uhr (WIAS-406)
FG Stochastische Systeme mit Wechselwirkung
Dr. Tim Hulshof, University of Technology, Niederlande:
Higher order corrections for anisotropic bootstrap percolation
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
Bootstrap percolation is a very simple model for growth from a random initial configuration on finite lattices. The model has many applications, for instance to model the spread of infections and to model magnets at low temperatures, to name two, but it is also interesting from a purely mathematical perspective. The model parameter has a critical value, at which the behaviour changes sharply. One interesting feature of bootstrap percolaton is a phenomenon called the ?bootstrap paradox? which relates to a big discrepancy between numerical and theoretical estimates of the critical value of bootstrap percolation models. I will discuss work in progress in which we give the most accurate theoretical estimate for the critical value of any bootstrap model to date, compare it with new numerical estimates, and show how it (tentatively) resolves the paradox. This talk is based on joint work with Hugo Duminil-Copin and Aernout van Enter, and on work with Robert Fitzner.

Veranstalter
WIAS Berlin
Dienstag, 21.02.2017, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Andrea Locatelli, Universität Potsdam:
Adaptation to noise parameters in non-parametric active learning
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
In this talk, I will present the non-parametric Active Learning classification setting already studied in Minsker (2012a) and show non-trivial strategies that can adapt to the smoothness of the regression function as well as the noise parameter and that require strictly weaker Assumptions than in previous work. We will first study the deterministic setting (i.e. one has access to the true value of the regression function) to build intuition, and then move on to the stochastic setting (noisy binary classification).

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WIAS Berlin
Donnerstag, 16.02.2017, 16:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Modelle der Photonik
Edward Arevalo, Pontificia Universidad Católica de Chile:
Solitary waves in the nonlinear Schrödinger equation with complex potentials
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Donnerstag, 16.02.2017, 09:00 Uhr (WIAS-406)
Halbleiterseminar
Dr. Annegret Glitzky, WIAS:
Analysis for a p(x)-Laplace thermistor model describing electrothermal effects in organic semiconductor devices
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

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Halbleiterseminar

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WIAS Berlin
Mittwoch, 15.02.2017, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Rüdiger Müller, WIAS:
Numerical analysis of coupled bulk-surface equations
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Coupled reaction di usion processes in the bulk and on the surface of some domain have recently attracted interest in di erent applications in engineering, chemistry and in biology like e.g. biological signal transduction. In this talk, an overview is given over numerical methods on boundary tted meshes as well as methods on un tted meshes. The corresponding a priori error analysis for suciently smooth solutions and uniformly re ned meshes is brie y discussed. Next, we study a stationary prototype problem and derive a posteriori error control for adaptive FEM. Emphasis is put on the case of only piecewise smooth domain boundaries and solutions of low regularity. A residual error estimator is derived which takes into account the approximation errors due to the nite element discretisation in space as well as the polyhedral approximation of the surface. Finally, a posteriori error estimates for the instationary and nonlinear problems are discussed.

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Berliner Oberseminar Nichtlineare partielle Differentialgleichungen (Langenbach-Seminar)

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WIAS Berlin
Mittwoch, 15.02.2017, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Lázsló Györfi, Budapest University of Technology and Economics:
The role of machine learning in the nonparametric prediction of time series
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Dienstag, 14.02.2017, 15:15 Uhr (WIAS-ESH)
Oberseminar Nonlinear Dynamics
Prof. Dr. Alessandro Torcini, Université de Cergy--Pontoise, France:
Death and rebirth of neural activity in sparse inhibitory networks
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
In this presentation, we clarify the mechanisms underlying a general phenomenon present in pulse-coupled heterogeneous inhibitory networks: inhibition can induce not only suppression of the neural activity, as expected, but it can also promote neural reactivation. In particular, for globally coupled systems, the number of firing neurons monotonically reduces upon increasing the strength of inhibition (neurons' death). However, the random pruning of the connections is able to reverse the action of inhibition, i.e. in a sparse network a sufficiently strong synaptic strength can surprisingly promote, rather than depress, the activity of the neurons (neurons' rebirth). Thus the number of firing neurons reveals a minimum at some intermediate synaptic strength. We show that this minimum signals a transition from a regime dominated by the neurons with higher firing activity to a phase where all neurons are effectively sub-threshold and their irregular firing is driven by current fluctuations. We explain the origin of the transition by deriving an analytic mean field formulation of the problem able to provide the fraction of active neurons as well as the first two moments of their firing statistics. The introduction of a synaptic time scale does not modify the main aspects of the reported phenomenon. However, for sufficiently slow synapses the transition becomes dramatic, the system passes from a perfectly regular evolution to an irregular bursting dynamics. In this latter regime the model provides predictions consistent with experimental findings for a specific class of neurons, namely the medium spiny neurons in the striatum.

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Oberseminar Nonlinear Dynamics

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Freie Universität Berlin
WIAS Berlin
Dienstag, 14.02.2017, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. Karsten Tabelow, WIAS Berlin:
MRI data models at low SNR
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
In this talk we elaborate the effect of low SNR on estimated parameters in models for neuroimaging data. We will present a new method for the local estimation of the noise parameter in the signal distribution and demonstrate how this can be used for improved estimation of model parameters. (joint work with Joerg Polzehl).

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WIAS Berlin
Donnerstag, 09.02.2017, 16:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Modelle der Photonik
Vinh Tho Ma, Technische Universität Berlin:
Numerical computation of eigenmodes in open waveguides with hp-adaptive FEM
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Donnerstag, 09.02.2017, 15:15 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Prof. Gert Lube und Ph. Schröder, Georg-August-Universität Göttingen:
Pressure-robust error estimates of exactly divergence-free FEM for time-dependent incompressible flows
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
The talk focusses on the analysis of a conforming finite element method for the time-dependent incompressible Navier--Stokes model. For divergence-free approximations, in a time-continuous formulation, we prove error estimates for the velocity that hold independently of both pressure and Reynolds number. A key aspect is the use of the discrete Stokes projection for the error splitting. Optionally, edge stabilization can be included in the case of dominant convection. Emphasizing the importance of conservation properties, the theoretical results are complemented with numerical simulations of vortex dynamics and laminar boundary layer flow.

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WIAS Berlin
Donnerstag, 09.02.2017, 14:00 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Prof. Michael Eikerling, Simon Fraser University, Kanada:
Theory and modeling of materials for electrochemical energy systems
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
The ever-escalating need for highly efficient and environmentally benign energy technology drives research on materials for fuel cells, supercapacitors, batteries, electrolysers, and other electrochemical systems. In this realm, physical-mathematical theory, modeling, and simulation provide increasingly powerful tools to unravel how multifunctional electrochemical materials come to life during self-organization, how they live and operate, e.g., by breathing in oxygen and breathing out water vapor or by shuttling ions across electrolytes, and how they age and fail because of normal wear-and-tear or improper use. The introductory part of the presentation will give a sweeping perspective of research forays in theory and modeling. Thereafter, two topics will be presented and discussed in detail: modeling approaches to study the interplay of interfacial charging phenomena, fluid flow, ion transport, and electrochemical reaction in nanoporous electrodes; and statistical physics-based modeling of degradation, aging, and failure in particle-based electrodes and fibrillar membranes.

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WIAS Berlin
Mittwoch, 08.02.2017, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Prof. Dr. Jürgen Sprekels, WIAS Berlin:
A new identification problem: optimizing the fractional order in a nonlocal evolution equation
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

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Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

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WIAS Berlin
Humboldt-Universität zu Berlin
Mittwoch, 08.02.2017, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Katharina Proksch, Universität Göttingen:
Multiscale scanning in inverse problems - With applications to nanobiophotonics
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Dienstag, 07.02.2017, 15:15 Uhr (WIAS-ESH)
Oberseminar Nonlinear Dynamics
Dr. Shalva Amiranashvili, WIAS Berlin:
Scattering of waves at solitons
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
Freie Universität Berlin
WIAS Berlin
Dienstag, 07.02.2017, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Benjamin Stemper, WIAS Berlin:
Efficient pricing in rough Bergomi
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

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WIAS Berlin
Dienstag, 07.02.2017, 10:15 Uhr (WIAS-ESH)
Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization
Dr. Sara Merino-Aceituno, Imperial College London, GB:
Kinetic theory to study emergent phenomena in biology: An example on swarming
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
Classical methods in kinetic theory are challenged when studying emergent phenomena in the biological and social sciences. New methodologies are needed to study the problems at hand which typically involve many agents that interact locally. The aim of this talk is to introduce the general framework of kinetic theory and some of the new challenges of applying it to biological systems. We illustrate it in the case of the so-called collective motion of self-propelled particles, like swarming of birds. Particularly, based on the Vicsek model, we study systems of agents (birds) that move at a constant speed while trying to align their body orientation with those of their neighbours. Starting from a particle description, we find the macroscopic dynamics.

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Joint Research Seminar on Nonsmooth Variational Problems and Operator Equations / Mathematical Optimization

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WIAS Berlin
Dienstag, 07.02.2017, 10:00 Uhr (WIAS-406)
Seminar Nichtlineare Optimierung und Inverse Probleme
Prof. Dr. Vincent Guigues, Fundação Getúlio Vargas (FGV), Brasilien:
Hypothesis testing and change point detection on controls of stochastic dynamical systems with independent state and observation noises belonging to spherical families
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Veranstalter
WIAS Berlin
Donnerstag, 02.02.2017, 14:00 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Dr. Joaquin Mura, Pontificia Universidad Católica de Chile, Chile:
An automatic method to estimate 3D Pulse Wave Velocity from 4D-flow MRI data
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
In this talk will be presented a novel method to automatically construct a continuous Pulse Wave Velocity map using 4D-Flow MRI data, based on the observation that in curved vessels, the propagation of velocity wavefronts do not necessarily follow perpendicular planes to some symmetry axis, but intricate shapes that strongly depends on the arterial morphology. This observation is considered to estimate continuous 1D PWV from velocities acquired with 4D-flow MRI data and projected back to 3D for better visualizations. This technique was assessed with in-silico and in-vitro phantoms, volunteers, and Fontan patients, showing a good agreement with expected values.

Veranstalter
WIAS Berlin
Donnerstag, 02.02.2017, 09:00 Uhr (WIAS-406)
Halbleiterseminar
Dr. Igor Yu. Popov, ITMO University, St. Petersburg, Russia:
Tunneling through periodic arrays of quantum dots and spectral problems
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
Periodic arrays of quantum dots in a constant magnetic field are considered. Two semi-infinite leads are attached to the system. We deal with tunneling through the system. Square and honeycomb lattices, single and double layers were investigated. Zero transmission for some values of the magnetic field is discussed.

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WIAS-Halbleiterseminar

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WIAS Berlin
Mittwoch, 01.02.2017, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. André Hänel, Leibniz Universität Hannover:
Spectral asymptotics for mixed problems and for crack problems on infinite cylinders
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
hier

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Berliner Oberseminar “Nichtlineare Partielle Differentialgleichungen” (Langenbach Seminar)

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
Mittwoch, 01.02.2017, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. Fabienne Compte, Université Paris Descartes:
Laguerre basis for inverse problems related to nonnegative random variables
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
I will present, through two main examples, the speci c properties of the Laguerre basis and show that it is a very convenient tool to solve estimation problems on R+. The rst example is the regression-convolution model: an estimator of the unknown underlying function is built in two steps (deconvolution step, regression step) which are explained and discussed. Then, a risk study is conducted, that shows as usual that a bias-variance tradeo must be performed. A model selection device is shown to solve this question. The second example concerns a simpler multiplicative model, for which a projection estimators of the density of the hidden variables are built and discussed. The speci c properties of the Laguerre basis with respect to these solutions are enhanced. Rates of convergence in relation with Sobolev-Laguerre spaces are presented. To conclude, several other problems solved with the Laguerre bases are listed.

Veranstalter
Humboldt-Universität zu Berlin
Universität Potsdam
SFB 649: Ökonomisches Risiko
WIAS Berlin
Dienstag, 31.01.2017, 15:15 Uhr (WIAS-ESH)
Oberseminar Nonlinear Dynamics
Dr. Matthias Wolfrum, WIAS Berlin:
Interfaces in a chain of coupled bistable elements
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
Freie Universität Berlin
WIAS Berlin
Dienstag, 31.01.2017, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Maurillio Danio Gutzeit, Universität Potsdam:
Separation rates in testing if two random graphs are based on the same model
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
We consider two inhomogenous Erdös- Renyi graphs on the same set of vertices and study the testing problem if the random edges of these graphs have the same underlying stochastic structure. In this framework, such a structure is typically represented by a symmetric n by n matrix with entries in [0,1] and zero diagonal: entry (i,j) is the probability of edge (i,j) to occur and the edges occur independently. Now, identifying the two graphs with such matrices P and Q, respectively, we want to describe the smallest Frobenius and spectral distances between P and Q such that there is a test whose total error probability does not exceed a given level. In particular, we focus on the dependence of this distances on the number n of vertices and the number M of collected samples per graph, i.e. observed adjacency matrices. The talk will exhibit the current state of ongoing joint research with Alexandra Carpentier, Ulrike von Luxburg and Debarghya Ghoshdastidar and it will include nonasymptic results from a minimax point of view as well asymptotic (with respect to n) problem- dependent results.

Veranstalter
WIAS Berlin
Dienstag, 31.01.2017, 10:00 Uhr (WIAS-406)
Seminar Nichtlineare Optimierung und Inverse Probleme
Prof. Dr. Masahiro Yamamoto, University of Tokyo, Japan:
Inverse problems and optimal control problems for fractional diffusion equations
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Veranstalter
WIAS Berlin
Donnerstag, 26.01.2017, 14:00 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
Dr. Franco Dassi, Politecnico di Milano, Italien:
The Virtual Element Method in three dimensions
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
The Virtual Element Method (VEM) is sharing a good degree of success in the recent years and its robustness and flexibility are already numerically provided for the two dimensional case. In three dimensions the results are currently restricted to the lowest order although the theory of this higher dimensional case is already developed in literature, see for instance [1, 2]. This talk is the first step towards a deeper numerical analysis of VEM in 3D [3]. After a first review of the scheme, we show a series of numerical results that validate this new method in three dimensions. To achieve this goal, we consider standard reaction-diffusion equations solved on several polyhedral meshes with different VEM order.
References: [1] L. Beirao da Veiga, F. Brezzi, A. Cangiani, G. Manzini, L. D. Marini, and A. Russo. Basic principles of virtual element methods. Mathematical Models and Methods in Applied Sciences, 23(01):199--214, 2013.
[2] L. Beirao da Veiga, F. Brezzi, L. D. Marini, and A. Russo. The hitchhiker's guide to the virtual element method. Mathematical Models and Methods in Applied Sciences, 24(08):1541--1573, 2014.
[3] L. Beirao da Veiga, F. Dassi, and A. Russo. Numerical investigations for three dimensional virtual elements of arbitrary order. to appear.

Veranstalter
WIAS Berlin
Donnerstag, 26.01.2017, 09:00 Uhr (WIAS-406)
Halbleiterseminar
Arbi Moses Badlyan, Techniche Universität Berlin:
On the port-Hamiltonian structure of the Navier-Stokes equations for reactive flows
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Weitere Informationen
Halbleiterseminar

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WIAS Berlin
Mittwoch, 25.01.2017, 15:15 Uhr (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. Olaf Klein, WIAS Berlin:
Uncertainty quantification for hysteresis operators and for a model for magneto-mechanical components
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
WIAS Berlin
Humboldt-Universität zu Berlin
Mittwoch, 25.01.2017, 10:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Dr. Fromont-Renoir Magalie, Université Rennes, Frankreich:
Family-wise separation rates for multiple testing
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstrakt
This joint work with Matthieu Lerasle (Univ. Paris-Saclay, France) and Patricia Reynaud- Bouret (Univ. Cote d'Azur, France) is devoted to the question of the theoretical evaluation of multiple testing procedures.Where as many rst kind error-related evaluation criteria have been de ned, as generali- zations or relaxations of the historical Family-Wise Error Rate (FWER), very few second kind error-related criteria have been proposed in the multiple testing literature. Starting from a parallel between some tests of multiple hypotheses and some tests of a single hypothesis, based on aggregation approaches known to lead to minimax adaptivity properties, we extend the notion of Separation Rate, at the core of the minimax theory for nonparametric single hypothesis tests, to the multiple testing eld.We thus introduce the notion of weak Family-Wise Separation Rate (wFWSR) and its stronger counterpart, the Family-Wise Separation Rate (FWSR), leading to an emergent minimax theory for multiple tests whose FWER is controlled. We present some illustrations in various classical Gaussian frameworks, that corroborate several expected results under particular conditions on the tested hypotheses, but also give more surprising results.

Veranstalter
WIAS Berlin
SFB 649: Ökonomisches Risiko
Universität Potsdam
Humboldt-Universität zu Berlin
Dienstag, 24.01.2017, 15:15 Uhr (WIAS-ESH)
Oberseminar Nonlinear Dynamics
Dr. Sergey Tikhomirov, Saint-Petersburg State University:
Depinning bifurcation in quasiperiodic media
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

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Freie Universität Berlin
WIAS Berlin
Dienstag, 24.01.2017, 15:00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Paolo Pigato, INRIA, Nancy, Frankreich:
Statistical estimation of the oscillating Brownian motion
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstrakt
The Oscillating Brownian Motion is a classical, simple example of stochastic differential equation with discontinuous diffusion coefficient. It behaves like a Brownian motion which changes variance parameter each time it crosses a fixed threshold. We consider here the problem of estimating the parameters of such process from discrete observations. We propose two natural consistent estimators, which are variants of the integrated volatility estimator and take the occupation times into account. We show the stable convergence of the renormalized errorsâ?? estimations toward some Gaussian mixture, possibly corrected by a term that depends on the local time. We then consider some applications to financial time series (in particular to volatility modeling), being the Oscillating Brownian Motion a simple way to account of volatility clustering and leverage effect. We compare our empirical results with other regime switching models. (Joint work with Antoine Lejay)

Veranstalter
WIAS Berlin
Donnerstag, 19.01.2017, 16:00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Modelle der Photonik
Franziska Bierbüsse, Humboldt-Universität zu Berlin:
Solitary solutions of the short pulse equation for a defocusing nonlinearity
mehr ... Veranstaltungsort
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Veranstalter
Humboldt-Universität zu Berlin
WIAS Berlin
Donnerstag, 19.01.2017, 11:00 Uhr (WIAS-406)
Analysis-Stochastik-Seminar