A quoi peut ressembler l'ensemble des zéros d'un polynômes réels en deux variables de degré fixé? D'un certain point de vue topologique, cette question a été résolu par Axel Harnack en 1876.

Plus de 100 ans plus tard, la méthode du patchwork combinatoire offre un outil simple et puissant pour répondre à cette question et à d'autres. Et ce sans même savoir ce qu'est un polynôme!

L'objectif de cet exposé sera de construire des courbes maximales par la méthode du patchwork.

In this talk we consider the four-waves spatially homogeneous kinetic equation arising in weak wave turbulence theory from the microscopic Fermi-Pasta-Ulam-Tsingou (FPUT) oscillator chains. This equation is sometimes referred to as the Phonon Boltzmann Equation. I will discuss the global existence and stability of solutions of the kinetic equation near the Rayleigh-Jeans (RJ) thermodynamic equilibrium solutions. This is a joint work with Pierre Germain (Imperial College London) and Joonhyun La (KIAS).

Je vous présenterai un résultat récent, en collaboration avec Benoît Grébert et Tristan Robert, concernant l'existence de tores invariants de dimension infinie pour les équations de Schrödinger non-linéaires sur le cercle (sans paramètres externes). En particulier, nous montrons l'existence de solutions presque-périodiques qui ne sont pas quasi-périodiques. Notre preuve repose notamment sur la théorie KAM et des effets régularisants dispersifs que je décrirai.

This talk will investigate the asymptotic behavior of evolution equations, including non-conservative ones, that are expected to converge to equilibrium at exponential speed. After reviewing some of the most classical methods used to derive this type of convergence, we will introduce an original approach to address this question. The key idea is to transform the problem into an equivalent conservative one, where solutions are probability measures, enabling the use of tools from optimal transport. Two examples in 1D -namely, the inhomogeneous heat equation with growth, and growth-fragmentation- will help us illustrate the method. This work is a collaboration with V. Calvez.

We consider the problem of learning a graph modeling the statistical relations of the d variables of a dataset with n samples. Standard approaches amount to searching for a precision matrix representative of a Gaussian graphical model that adequately explains the data. However, most maximum likelihood-based estimators usually require storing the d^2 values of the empirical covariance matrix, which can become prohibitive in a high-dimensional setting. In this talk, we adopt a “compressive” viewpoint and aim to estimate a sparse precision matrix from a sketch of the data, i.e., a low-dimensional vector of size m≪d^2 carefully designed from the data using nonlinear random features (e.g., rank-one projections). Under certain spectral assumptions, we show that it is possible to recover the precision matrix from a sketch of size m=Ω((d+2k)log(d)), where k is the maximal number of edges of the underlying graph. These information-theoretic guarantees are inspired by the compressed sensing theory. We investigate the possibility of achieving practical recovery with an iterative algorithm based on the graphical lasso, viewed as a specific denoiser. We compare our approach and the graphical lasso on synthetic datasets, demonstrating its favorable performance even when the dataset is compressed. Joint work with : Titouan Vayer, Rémi Gribonval and Paulo Gonçalves.

It is known that a manifold diffeomorphic to a K3 surface admits an almost toric fibration (ATF). However, given a specific symplectic form on a K3 it is unclear if it admits an almost toric fibration with lagrangian fibres for the given form. In this talk, we prove that when a Kähler K3 surface admits a Type III Kulikov degeneration with a symplectic form taming the complex structure, the symplectic form admits an ATF whose base is the intersection complex of the degenerate fibre. Furthermore, we shall show that a smooth anti-canonical hypersurface in a smooth toric Fano threefold, equipped with a toric Kähler form, admits such a symplectic Kulikov model.

This is based on joint work with Yoel Groman.