# séminaire-edp-BBT

Organisé à tour de rôle par des membres du BCAM, de l’IMB et de l’IMT, le séminaire Bilbao-Bordeaux-Toulouse (BBT) est centré autour des thématiques du GdR Analyse des EDP, avec une ouverture vers l’analyse numérique et les probabilités. Il a lieu un mardi par mois, à 11h et est diffusé en visio-conférence avec, autant que possible, l’orateur présent sur l’un des trois sites.

17 Mai 2022 (horaire exceptionnel : 14h-15h)

Patrick Gérard (Paris-Saclay / Orsay) –  On a derivative nonlinear Schrödinger equation on the Hardy space of the line

Abstract : We introduce a nonlinear Schrödinger equation on the line, with a mass critical nonlocal cubic nonlinearity  of DNLS type, which conserves the Hardy property of a Fourier transform supported in the  positive half line. We identity a Lax pair  for this equation, and we use this structure for studying  multisoliton solutions. This a joint work with Enno Lenzmann (Basel).

Site organisateur : Toulouse
Lien zoom : https://zoom.us/j/92233044531?pwd=MFA5N0RXLzRGSHY4aVZ1OFM2WUw3Zz09

ID de réunion : 922 3304 4531
Code secret : 4W2ydd

5 Avril 2022

Antti Kupiainen (Helsinki) – Renormalisation group and SPDEs

Abstract: Non-linear diffusive PDEs driven by space-time white noise require infinite renormalisations to be well posed. I will discuss why this is the case and how the renormalisations can be found by using an idea from quantum field theory, the renormalisation group.

Site organisateur : Bilbao
Lien zoom : https://us06web.zoom.us/j/99649860282?pwd=SE0vemtYMFlwbFBNTXQyOTBONG0vZz09

15 Mars 2022

Didier Bresch (Chambéry) – Mean field limits and singular kernels: some recent advances.
Abstract: In this talk, I will present mathematical justifications for mean field limits with singular nuclei based on the control of appropriate weights. These weights must be dynamic and fully relevant to the problem under consideration. We will explain some recent results obtained with Pierre-Emmanuel Jabin (Penn-State) and initially with Z. Wang (Peking Univ) then in a second time with J. Soler (Granada Univ.) respectively around systems of order 1 and then around systems of order 2. This idea of ​​well-adapted dynamical weights finds for us its origin in a joint work with P.-E. Jabin on compressible Navier-Stokes.

Site organisateur : Bordeaux
Lien zoom: https://u-bordeaux-fr.zoom.us/j/85496437138

15 Juin 2021

Nicolas Burq (Orsay) –  Almost sure scattering for the one dimensional nonlinear Schrödinger equation

Abstract:  We consider the one-dimensional nonlinear Schrödinger equation with a nonlinearity of degree $p>1$.  On compact manifolds many probability measures are invariant by the flow of the  linear Schrödinger equation (e.g. Wiener measures), and it is sometimes possible to modify them suitably and get  invariant (Gibbs measures)  or  quasi-invariant measures for the non linear problem. On $\mathbb{R}^d$, the large time dispersion shows that the only invariant measure is the $\delta$ measure on the trivial solution $u =0$, and the good notion to track is whether the non linear evolution of the initial measure is well described by the linear (non trivial) evolution. This is precisely what we achieve in this work.  We exhibit measures on the space of initial data for which we describe the non trivial evolution by the linear Schrödinger flow and we show that their nonlinear evolution is absolutely continuous with respect to this linear evolution. Actually, we give precise (and optimal) bounds on the Radon-Nikodym derivatives of these measures with respect to each other and we characterise their $L^p$ regularity.  We deduce from this precise description the global well-posedness of the equation for $p>1$ and scattering for $p>3$ (actually even for $1<p \leq 3$, we get a dispersive property of the solutions and exhibit an almost sure polynomial decay in time of their $L^{p+1}$ norm). To the best of our knowledge, it is the first occurence where the description of quasi-invariant measures allows to get quantitative asymptotics (here scattering properties or decay) for the nonlinear evolution. This is a joint work with L. Thomann (Université de Lorraine)

Site organisateur : Bordeaux (David Lannes, Laurent Michel)

Lien zoom https://u-bordeaux-fr.zoom.us/j/81041813364 (ID de réunion : 810 4181 3364)

11 Mai 2021 – Attention horaire exceptionnel : 15h

Eugenia Malinnikova (Stanford/St-Petersbourg) – On Dirichlet Laplace eigenfunctions in Lipschitz domains with small Lipschitz constant.
Abstract: We consider  bounded domains in the Euclidean space with Lipschitz boundary and locally small Lipschitz constant. We proof the sharp upper bound for the area of the nodal sets of Dirichlet Laplace eigenfunctions in such domains. One of our tools is the analysis of the frequency function of a harmonic function vanishing on a part of the boundary.
The talk is based on a joint work with A. Logunov, N. Nadirashvili, and F. Nazarov.
Site organisateur : Bilbao (Aingeru Fernandez Bertolin, Luis Vega)

Meeting ID: 966 7681 5209

Passcode: 027793

6 Avril 2021

Pierre Degond (IMT) – Topological states in collective dynamics

Résumé : States of matter are characterized by different types of order. Recently, a new notion of order, popularized by the 2016 physics nobel prizes, has emerged : that of topological order. It refers to the global rigidity of the system arising from topological constraints. Recently, topological states has been shown to exist in collective dynamics, which describes systems of self-propelled particles. In this work, we consider a system of self-propelled solid bodies interacting through local full body alignment proposed in a joint work with A. Frouvelle, S. Merino-Aceituno and A. Trescases. In the large-scale limit, this system can be described by hydrodynamic equations with topologically non-trivial explicit solutions. At the particle level, these solutions undergo topological phase transitions towards trivial flocking states. Numerically we show that these transitions require the system to pass through a phase of disorder. To our knowledge, it is the first time that a hydrodynamic model guides the design of topologically non-trivial states and allows for their quantitative analysis and understanding. On the way, we will raise interesting mathematical questions underpinning the analysis of collective dynamics systems.
Joint work with Antoine Diez and Mingye Na (Imperial College London)

Site organisateur : Toulouse (J.M. Bouclet, R. Duboscq, M. Maris, A. Trescases)

Video of the talk : https://www.youtube.com/watch?v=EIBfmIJbnFo