Events & Teaching

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May 10 – 12, 2016 (WIAS-ESH)
Workshop/Konferenz: WIAS-PGMO Workshop on Nonsmooth and Stochastic Optimization with Applications to Energy Management
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
WIAS Berlin
Tuesday, 10.05.2016, 16.00 Uhr (WIAS-405-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Prof. L. Berlyand, Pennsylvania State University, USA:
Phase field model of cell motility: Sharp interface limit and traveling waves
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Abstract
Phase field models are very ecient in computational studies of moving deformable in- terfaces. We will present a phase field system that models the motion of a eukaryotic cell on a substrate and investigate the dependence of this motion on key physical parameters. This system consists of two PDEs: the Allen-Cahn equation for the scalar phase eld function coupled with a vectorial parabolic equation for the orientation of the actin lament network. Two key features of this system are (i) the gradients coupling and (ii) volume preservation constraints. We pass to the sharp interface limit (SIL), which reduces the system to a single scalar equation and show that the motion of the cell boundary is the mean curvature motion modied by a novel nonlinear term. Numerical and analytical studies of the SIL equation reveal the existence of two distinct regimes of the physical parameters. The subcritical regime was studied numerically and analytically by my Ph. D. student M. Mizuhara. Our main focus is the supercritical regime. Here we established surprising features of the motion of the interface such as discontinuities of velocities, hysteresis in the 1D model, instability of the circular shape, rise of asymmetry in the 2D model, and existence of non-trivial traveling waves. Because of features (i)-(ii), classical comparison principle techniques do not apply to this system. Furthermore, the system can not be written in a form of gradient ow, which is why ô-convergence techniques also can not be used. Instead, our derivation of SIL is based on a special asymptotic ansatz. This is joint work with V. Rybalko and M. Potomkin.

Further Informations
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics

Host
WIAS Berlin
Wednesday, 11.05.2016, 10.00 Uhr (WIAS-406)
Forschungsseminar Mathematische Statistik
Prof. S. Lee, Seoul National University:
Oracle estimation of a change point in high dimensional quantile regression
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
In this paper, we consider a high dimensional quantile regression model where the sparsity structure may differ between the two sub-populations. We develop ?1-penalized estimators of both regression coefficients and the threshold parameter. Our penalized estimators not only select covariates but also discriminate between a model with homogeneous sparsity and a model with a change point. As a result, it is not necessary to know or pretest whether the change point is present, or where it occurs. Our estimator of the change point achieves an oracle property in the sense that its asymptotic distribution is the same as if the unknown active sets of regression coefficients were known. Importantly, we establish this oracle property without a perfect covariate selection, thereby avoiding the need for the minimum level condition on the signals of active covariates. Dealing with high dimensional quantile regression with an unknown change point calls for a new proof technique since the quantile loss function is non-smooth and furthermore the corresponding objective function is non-convex with respect to the change point. The technique developed in this paper is applicable to a general M-estimation framework with a change point, which may be of independent interest. The proposed methods are then illustrated via Monte Carlo experiments and an application to tipping in the dynamics of racial segregation.

Host
Humboldt-Universität zu Berlin
SFB 649: Ökonomisches Risiko
Universität Potsdam
WIAS Berlin
Thursday, 12.05.2016, 10.00 Uhr (WIAS-406)
Analysis-Stochastik-Seminar
Dr. H. Stephan, WIAS Berlin:
Solving the Hamilton--Jacobi equation by the Hopf--Lax method
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Host
WIAS Berlin
Thursday, 12.05.2016, 14.00 Uhr (WIAS-405-406)
Seminar Numerische Mathematik
Prof. W. Dreyer, WIAS Berlin:
Subtle properties of the barycentric velocity in mixture models
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Raum: 405/406

Abstract
In a single substance the barycentric velocity is a primitive concept while in a mixture of several constituents it is defined via the velocities of the constituents. In both cases the evolution of the barycentric velocity is determined by the balance equations of mass and momentum. There are many inconsistencies in constitutive models in the field of the barycentric velocity. In this lecture we illustrate these inconsistencies by means of three practical ex- amples. The most prominent example concerns an incompressible material with thermal expansion. A further example is provided by a liquid-vapor phase transition. Finally we study the plating process of two copper electrodes immersed in an electrolyte.

Host
WIAS Berlin
Tuesday, 17.05.2016, 15.15 Uhr (FU Berlin)
Oberseminar Nonlinear Dynamics
B. de Rijk, Universiteit Leiden, Niederlande:
Factorization of the Evans function via the Riccati transformation
more ... Location
Freie Universität Berlin, Arnimallee 3, 14195 Berlin, Hinterhaus, Raum 130

Abstract
In the spectral stability analysis of pattern solutions, the presence of a small parameter can reduce the complexity of the linear stability problem. The spectrum of the linearization about certain type of patterns is given by the zero set of an analytic function, the so-called Evans function. Our reduction method yields a factorization of the Evans function in accordance with the scale separation induced by the small parameter. For some specific equations this product structure has yet been established by geometric arguments. Our analytic method formalizes and generalizes the factorization procedure. The main tool for the reduction is the Riccati transformation. We employ our techniques to study the stability of stationary, spatially periodic pulse patterns to a general class of singularly perturbed reaction-diffusion systems. Our Evans-function analysis complemented with a careful analysis of the spectral curve attached to the origin leads to explicit conditions for nonlinear diffusive stability.

Further Informations
Oberseminar Nonlinear Dynamics

Host
WIAS Berlin
Freie Universität Berlin
Wednesday, 18.05.2016, 15.15 Uhr (WIAS-ESH)
BERLINER OBERSEMINAR Nichtlineare partielle Differentialgleichungen (Langenbach-Seminar)
A. Stephan, Humboldt-Universität zu Berlin:
On operator-norm estimates for approximations of solutions of evolution equations using the Trotter product formula
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Further Informations
Berliner Oberseminar ``Nichtlineare Partielle Differentialgleichungen'' (Langenbach Seminar)

Host
WIAS Berlin
Humboldt-Universität zu Berlin
Tuesday, 24.05.2016, 10.15 Uhr (WIAS-406)
Seminar Nichtlineare Optimierung und Inverse Probleme
D. Groh, Humboldt-Universität zu Berlin:
Bilevel optimization method for the blind deconvolution problem with total variation regularization
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Host
WIAS Berlin
Wednesday, 25.05.2016, 10.00 Uhr (WIAS-ESH)
Forschungsseminar Mathematische Statistik
Y. Maximov, Russian Academy of Sciences, Russische Föderation:
Structured semi-definite programming with applications to non-Gaussian component analysis
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
Semi-definite programming (SDP) is a popular tool for approximation of non-convex quadratic problems arises in multiple statistical and computer science problems. Known to be worst-case optimal SDP is often dominated on well-structured (practical) problems by domain specific methods and heuristics. Yet another problem of SDP is a slow computational time makes it hardly applicable for huge-scale problems. In this talk we try to incorporate problem structure in the semi-definite dual to contribute both decrease computational time and improve approximation guarantees. A major application the proposed structured semi-definite programming is a novel convex projection based method in Non-gaussian component analysis (NGCA) especially efficient for projecting sparse and low-rank high-dimensional data. The talk is based on the joint research results with Yu. Nesterov (CORE/UCL, Belgium) and V. Spokoiny (WIAS, Berlin)

Further Informations
Forschungsseminar ``Mathematische Statistik''

Host
Humboldt-Universität zu Berlin
Universität Potsdam
WIAS Berlin
Wednesday, 01.06.2016, 15.15 Uhr (WIAS-ESH)
BERLINER OBERSEMINAR Nichtlineare partielle Differentialgleichungen (Langenbach-Seminar)
Dr. M. Heida, WIAS Berlin:
Stochastic homogenization of rate-independent systems
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Further Informations
Berliner Oberseminar ``Nichtlineare Partielle Differentialgleichungen'' (Langenbach Seminar)

Host
WIAS Berlin
Humboldt-Universität zu Berlin
Thursday, 02.06.2016, 14.00 Uhr (WIAS-ESH)
Seminar Numerische Mathematik
N. Alia, WIAS Berlin:
Introduction to the project about optimal control of ladle stirring
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Abstract
The doctoral subject Öptimal control of ladle stirring" is part of the MIMESIS project led by D. Hömberg and funded by the EU through its European Industrial Doctorate initiative. In order to produce high performance steel, refinement metallurgical processes are used by steelmakers. One example of these processes are vacuum tank degassers, where the liquid steel is contained in a vacuum ladle and is subject to an inert gas flow in order to evacuate impurities such as gaseous Hydrogen particles. The control of this process is a main issue for steel plants which aim at a competitive production of high performance steel. One possibility to achieve this is to consider the vibration measured on the ladle wall and to link it with the velocity of the flow happening inside the ladle. This needs the mathematical modeling and computation of mainly two phenomena : the turbulent multiphase flow inside the ladle and the fluid/structure interaction. Then, experiments have to be carried out in order to derive an operational index which can predict the steel quality based on vibration measurements and simulations. A general introduction to this doctoral work is presented in this seminar.

Host
WIAS Berlin
Tuesday, 07.06.2016, 16.00 Uhr (WIAS-406)
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics
Dr. A. Naumov, Lomonosov Moscow State University, Russische Föderation:
Extreme singular values of random matrices
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, 4. Etage, Weierstraß-Hörsaal (Raum: 406)

Abstract
In my talk I will discuss the bounds for the smallest and largest singular values of different random matrix ensembles. These bounds play a crucial role in many limit theorems in Random matrix theory. Some applications to numerical mathematics and data analysis will be given as well. The talk will be partially based on joint results with F. Götze and A. Tikhomirov.

Further Informations
Seminar Modern Methods in Applied Stochastics and Nonparametric Statistics

Host
WIAS Berlin
Wednesday, 08.06.2016, 15.15 Uhr (WIAS-ESH)
BERLINER OBERSEMINAR Nichtlineare partielle Differentialgleichungen (Langenbach-Seminar)
Dr. S. Nesenenko, Technische Universität Berlin:
A Fintzpatrick function approach in the homogenization of elasto-viscoplastic models with isotropic hardening effects
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Further Informations
Berliner Oberseminar ``Nichtlineare Partielle Differentialgleichungen'' (Langenbach Seminar)

Host
WIAS Berlin
Humboldt-Universität zu Berlin
July 14 – 15, 2016 (WIAS-ESH)
Workshop/Konferenz: New Methods in Extension Theory applied to Quantum Mechanics
more ... Location
Weierstraß-Institut, Mohrenstr. 39, 10117 Berlin, Erdgeschoss, Erhard-Schmidt-Hörsaal

Host
WIAS Berlin