Upcoming Events

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

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

Abstract
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.

Host
WIAS Berlin
Saturday, 03.08.2019, 09:00 (TU-Berlin)
6th International Conference on Continuous Optimization
6th International Conference on Continuous Optimization
more ... Location
Technische Universität, Straße des 17. Juni 135, 10623 Berlin, Main building (H)

Further Informations
https://iccopt2019.berlin/

Host
WIAS Berlin
Technische Universität Berlin
Freie Universität Berlin
Humboldt-Universität zu Berlin
Mathematical Optimization Society
Friday, 02.08.2019, 15:15 (WIAS-ESH)
MATHEON Special Guest Lecture
Prof. Y.-X. Yuan, Chinese Academy of Sciences, China:
Efficient optimization algorithms for large scale data analysis
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
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.

Further Informations
Special Guest Lecture

Host
WIAS Berlin
Thursday, 27.06.2019, 16:00 (WIAS-ESH)
Forschungsseminar Mathematische Modelle der Photonik
J.-P. Koester, Ferdinand-Braun-Institut, Berlin:
Traveling wave model based simulation of tunable multi-wavelength diode laser and MOPA systems
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

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

Abstract
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 it 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.

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 12.06.2019, 15:15 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
F. Della Porta, Max-Planck-Institut für Mathematik Leipzig:
The nonlocal Cahn--Hilliard--Hele--Shaw system with regular and singular potential
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
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.

Host
Humboldt-Universität zu Berlin
WIAS Berlin
Wednesday, 05.06.2019, 15:00 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. P. 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)
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
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.

Further Informations
Berliner Oberseminar “Nichtlineare partielle Differentialgleichungen" (Langenbach-Seminar)

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

Abstract
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.

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

Abstract
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.

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

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

Abstract
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.

Host
WIAS Berlin
Wednesday, 29.05.2019, 15:00 (WIAS-ESH)
Berliner Oberseminar „Nichtlineare partielle Differentialgleichungen” (Langenbach-Seminar)
Dr. P. 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)
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
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.

Further Informations
Berliner Oberseminar “Nichtlineare partielle Differentialgleichungen" (Langenbach-Seminar)

Host
WIAS Berlin
Humboldt-Universität zu Berlin
Wednesday, 29.05.2019, 10:00 (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Prof. D. Belomestny, Universität Duisburg-Essen:
Density deconvolution under general assuptions on measurement error distribution
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
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).

Host
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Tuesday, 28.05.2019, 15:15 (WIAS-ESH)
Oberseminar Nonlinear Dynamics
S. Eydam, WIAS Berlin:
Mode-locked solutions and coherence echoes in systems of globally-coupled phase oscillators
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
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.

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

Abstract
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.

Host
WIAS Berlin
Tuesday, 28.05.2019, 10:15 (WIAS-406)
Seminar Nichtlineare Optimierung und Inverse Probleme
Dr. B. Sprungk, Georg-August-Universität Göttingen:
Noise-level robust sampling methods for Bayesian inverse problems
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Host
WIAS Berlin