# Joint Analysis Seminar

The Joint Analysis Seminar takes place Tuesday, 10:30 -11:30 am, in room 008, Pontdriesch 14-16. “1950|008 SeMath”

## Upcoming Talks

07-17-2018, 10:30 AM

**Simon Foucart** (Texas A&M University):**Semidefinite programming in approximation theory: two examples**

07-10-2018, 10:30 AM

**Dario Trevisan** (Universität Pisa):**A PDE approach to random matching problems**

06-05-2018, 10:30 AM

**Paolo Bonicatto** (Universität Basel):**Untangling of trajectories for non-smooth vector fields and Bressan’s compactness conjecture**

Given d≥1, T>0 and a vector field **b**: [0,T] × ℝ^{d} → ℝ^{d},
we study the problem of uniqueness of weak solutions to the associated transport equation
∂_{t}u + **b**·∇u = 0 where u: [0,T] × ℝ^{d} → ℝ
is an unknown scalar function. In the classical setting, the method of characteristics is available
and provides an explicit formula for the solution of the PDE, in terms of the flow of the vector field
**b**. However, when we drop regularity assumptions on the velocity field, uniqueness is in
general lost.
In the talk we will present an approach to the problem of uniqueness based on the concept of Lagrangian
representation. This tool allows to represent a suitable class of vector fields as superposition of
trajectories: we will then give local conditions to ensure that this representation induces a partition
of the space-time made up of disjoint trajectories, along which the PDE can be disintegrated into a
family of 1-dimensional equations. We will finally show that if **b** is locally of class BV
in the space variable, the decomposition satisfies this local structural assumption: this yields in
particular the renormalization property for nearly incompressible BV vector fields and thus gives
a positive answer to the (weak) Bressan's Compactness Conjecture. This is a joint work with S. Bianchini.

04-24-2018, 10:30 AM

**Enno Lenzmann** (Universität Basel):**Energy-critical half-wave maps: solitons and Lax pair structure**

The eneryg-critical half-wave maps equation arises as a classical continuum limit of Calogero-Moser and Haldane-Shastry type spin systems in one space dimension. In this talk, I will discuss some essential features such as the complete classification of traveling solitary waves with finite energy, by using a close connection to minimal surfaces with free and non-free boundary conditions. Furthermore, I will present a recently found Lax pair structure and we explain its potential applications to the dynamics of the half-wave maps equation. Finally, I will mention some open problems. This talk is based on joint work with P. Gérard (Orsay) and A. Schikorra (Pittsburgh).

02-20-2018, 10:30 AM

**Filip Rindler** (University of Warwick):**Rademacher’s Theorem, Cheeger’s conjecture and PDEs for measures**

01-23-2018, 10:30 AM

**Herbert Koch** (University of Bonn):**The renormalized nonlinear wave equation in 2d with additive white noise**

By the classical uniformization theorem, every smooth Riemann surface is conformally diffeomorphic to a surface of constant curvature. What happens if the smooth Riemannian metric is replaced by a non-smooth distance? Does the so obtained metric surface still admit parametrizations with good geometric and analytic properties? Such questions have been widely studied in the field of Analysis on metric spaces and are important for example in view of applications to Geometric Group Theory. I will explain how one can use recently established existence and regularity of area and energy minimizing discs in metric spaces to obtain canonical parametrizations of metric surfaces. In particular, we obtain a new and conceptually simple proof of a well-known theorem of Bonk and Kleiner on the existence of quasisymmetric parametrizations of linearly locally connected, Ahlfors 2-regular metric 2-spheres. Joint work with Alexander Lytchak.

12-19-2017, 10:30 AM

**Daniel Cremers** (TU München):**Convex relaxation methods for computer vision**

Numerous problems in computer vision and image analysis can be solved by optimization methods. Yet, many classical approaches correspond to non-convex energies giving rise to suboptimal solutions and often strong dependency on appropriate initialization. In my presentation, I will show how problems like stereo or multiple view reconstruction and 3D shape matching can be tackled by means of convex relaxation methods which allow to efficiently compute globally optimal or near-optimal solutions. The arising large-scale convex problems can be solved by means of provably convergent primal-dual algorithms. They are easily parallelized on GPUs providing high-quality solutions in acceptable runtimes.

11-21-2017, 10:30 AM

**Stefan Wenger** (Université de Fribourg):**Finding good parametrizations for metric surfaces**

07-11-2017, 10:30 AM

**Jean V. Bellissard** (Westfälische Wilhelms-Universität, Münster, retired from the Georgia Institute of Technology):**Special Joint Analysis Seminar (Mathematics & Physics):
A Toy Model for Viscosity**

A short review of the temperature behavior of liquids viscosity will be provided. The concept of anankeon and as a new degree of freedom will be described and its relation with elastic degrees of freedom discussed. Then a simplistic solvable model, based on a Stochastic Markov dynamics, will be proposed and the solution explained and discussed. One consequence is the prediction that in a certain subclass of liquids, there is a bifurcation leading to a new time scale, the Maxwell time, which is liable to explains the exponential increase of the viscosity near the liquid-solid transition. A comparison with numerical simulations using molecular dynamics will be discussed.

06-13-2017, 10:30 AM

**Helmut Abels** (Universität Regensburg):**Diffuse Interface Models for Two-Phase Flows of Incompressible Fluids and Their Sharp Interface Limits**

We discuss so-called "diffuse interface models" for the flow of two viscous incompressible Newtonian fluids in a bounded domain. Such models were introduced to describe the flow when singularities in the interface, which separates the fluids, (droplet formation/coalescence) occur. The fluids are assumed to be macroscopically immiscible, but a partial mixing in a small interfacial region is assumed in the model. Moreover, diffusion of both components is taken into account. We discuss analytic results on well-posedness and qualitative behavior for some of these models and relation to classical sharp interface models, when the small parameter related to the thickness of the interface tends to zero.

05-23-2017, 10:30 AM

**Etienne Sandier** (Université Paris 12):**Lower bound for energy growth of locally minimizing solutions of -Δu=u(1-|u| ^{2}) for u: ℝ^{3}→ℝ^{2}**

05-09-2017, 10:30 AM

**Guido de Philippis** (SISSA, Trieste):**On the structure of measures satisfying a PDE constraint**

After a general introduction concerning the study of the interplay between PDE constraint and concentration/oscillation, I will present a general structure theorem for the singular part of Radon measure satisfying a PDE constraint. I will then present some applications.

05-02-2017, 10:30 AM

**David Gross** (Universität zu Köln):**Low rank matrix recovery, the Clifford group, and some quantum mechanics**

02-07-2017, 02:15 PM

**Giuseppe Savare** (University of Pavia):**Singular perturbation of gradient flows and rate-independent evolution problems**

We will present some new results concerning rate-independent limits of singularly perturbed gradient flows. A particularly important case arises when the total variation of the approximating curves is not uniformly bounded and one has to recover a limit by combining Kuratowski convergence of the graphs with a variational description of the energy dissipation. This approach leads to a new notion of dissipative solution to a family of stationary problems parametrized by the time variable. (In collaboration with Virginia Agostiniani and Riccarda Rossi)

12-06-2016, 02:15 PM

**John M. Ball** (University of Oxford):**The mathematics of liquid crystals**

11-15-2016, 02:15 PM

**Qiji J. Zhu** (Western Michigan University):**Variational methods in the presence of symmetry**

10-25-2016, 02:15 PM

**Gero Friesecke** (TU München):**Inferring atomic structure from X-ray diffraction patterns**

# Post Graduate Seminar

The group seminar takes place Tuesdays, 10:00 - 11:30 am in room 208 (Pontdriesch 10..

## Upcoming Talks in Post Graduate Seminar

07-19-2018, 03:40 PM

**Nadav Cohen** (Princeton Institute of Advances Studies):**On Optimization in Deep Learning: Implicit Acceleration by Overparameterization**

Deep learning refers to a class of statistical learning models that is enjoying unprecedented success in recent years, powering state of the art technologies in numerous application domains. However, despite the vast scientific and industrial interest it is drawing, our theoretical understanding of deep learning is partial at best. In this talk I will outline my perspective on the fundamental questions in deep learning theory, highlighting the difference from classical machine learning. I will then focus on the question of optimization, where depth (non-convexity) is believed to be an impediment. In stark contrast with conventional wisdom, I will show that, sometimes, increasing depth can speed up optimization. This result was recently derived with Sanjeev Arora and Elad Hazan (to appear in the proceedings of ICML 2018; arXiv preprint: https://arxiv.org/pdf/1802.06509.pdf).

Lists of Passed Talks