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# La semaine de l’IMB

La semaine de l’IMB recense l’ensemble des événements de la semaine en cours

• Le 2 mars 2021 à 10:00
• Séminaire de Théorie Algorithmique des Nombres
Online
Explicit construction and parameters of projective toric codes

Toric codes, introduced by Hansen in 2002, generalize (weighted) Reed-Muller codes on other toric varieties than projective spaces. They consist of evaluation codes of monomials at tuples of non-zero coordinates, which correspond to the points on the dense torus contained in the associated toric variety. Our aim is to ‘projectivise’ these codes, in the same spirit that turns a Reed-Muller codes into a projective one: we consider codes obtained by evaluating global sections on the whole set of the rational points of a toric variety. We focus on simplicial toric varieties, which come with a nice quotient description, and we give an explicit construction of projective codes on them, as well as a combinatorial way to determine their parameters. ‘Projectivizing’ toric codes opens new possibilities of getting codes with excellent parameters, by extending some champion classical toric codes geometrically.

• Le 2 mars 2021 à 11:15
• Séminaire de Physique Mathématique - EDP
Visio
Quantum Confinement induced by Dirac operators with anomalous magnetic $delta$-shell interactions.
Abstract: Let $\Omega$ be a bounded domain and $\upsilon\in\mathbb{R}$. I will consider the coupling $\mathcal{H}_{\upsilon}=\mathcal{H}+ V_\upsilon$, where $\mathcal{H}$ is the free Dirac operator in $\mathbb{R}^3$ and $V_\upsilon= i\upsilon\beta(\alpha\cdot \mathit{N})\delta_{\partial\Omega}$ is the anomalous magnetic $\delta$-interactions potential. In the first instance, assuming that $\upsilon^2 eq 4$ and under some regularity assumption on the domain $\Omega$, we prove that $\mathcal{H}_{\upsilon}$ is self-adjoint and its domain is included in the Sobolev space $\mathit{H}^{1}(\mathbb{R}^3\setminus \partial\Omega)^4$. Moreover, a Krein-type resolvent formula and a Birman-Schwinger principle are obtained, and several qualitative spectral properties of $\mathcal{H}_{\upsilon}$ are given. Finally, we study the self-adjoint realization of $\mathcal{H}_{\upsilon}$ in the case $\upsilon^2=4$. In particular, if $\Omega$ is $\mathit{C}^1$-smooth, we then show that $\mathcal{H}_{\upsilon}$ is essentially self-adjoint and the domain of the closure is not included in any Sobolev space $\mathit{H}^{s}(\mathbb{R}^3\setminus \partial\Omega)^4$, for all $s>0$. In addition, we show that $\overline{\mathcal{H}_{\pm2}}$ generates confinement and prove the existence of embedded eigenvalues on the essential spectrum of $\overline{\mathcal{H}_{\pm2}}$.
• Le 4 mars 2021 à 11:00
• Séminaire Images Optimisation et Probabilités
Salle de Conférences
François-Pierre Paty (CREST, ENSAE Paris)
Regularizing Optimal Transport through Regularity Constraints
Optimal transport (OT) suffers from the curse of dimensionality. Therefore, OT can only be used in machine learning if it is substantially regularized. In this talk, I will present a new regularization of OT which leverages the regularity of the Brenier map. Instead of considering regularity as a property that can be proved under suitable assumptions, we consider regularity as a condition that must be enforced when estimating OT. From a statistical point of view, this defines new estimators of the OT map and 2-Wasserstein distance between arbitrary measures. From an algorithmic point of view, this leads to an infinite-dimensional optimization problem, which, when dealing with discrete measures, can be rewritten as a finite-dimensional separately-convex problem. I will finish by sharing some recent ideas on how to speed up the algorithms. The talk is based on some joint work with Marco Cuturi and Alexandre d'Aspremont.
• Le 5 mars 2021 à 10:00
• Soutenance de thèse
Salle de Conférences
Gaël GUILLOT
Sujet :'Méthodes d'agrégation et désagrégation de programmes linéaires en nombres entiers'. Directeur de thèse : François Clautiaux, co-directeur : Boris Detienne.

• Le 5 mars 2021 à 16:00
• Séminaire de Théorie des Nombres
Visio
Türkü Özlüm Çelik (Simon Fraser University, Vancouver)
KP equation in Symbolic, Numerical and Combinatorial Algebraic Geometry
The Kadomtsev-Petviashvili (KP) equation is a partial differential equation that describes nonlinear wave moves. It is known that algebro-geometric approaches to the KP equation provide solutions coming from a complex algebraic curve, in terms of the Riemann theta function associated with the curve. Reviewing this relation, I will introduce an algebraic object and discuss its geometric features: the so-called Dubrovin threefold of a complex algebraic curve, which parametrizes the solutions. Mentioning the relation of this threefold with the classical algebraic geometry problem, namely the Schottky problem, I will report a procedure that isvia the threefold and based on numerical algebraic geometric tools, which can be used to deal with the Schottky problem from the lens of computations. I will finally focus on the geometric behaviour of the threefold when the underlying curve degenerates. This is joint work with Daniele Agostini and Bernd Sturmfels.

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