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Aujourd'hui

Régularité et comportement en temps grand des solutions spatialement homogènes de l’équation de Landau de la physique des plasmas

Salle W de l'ENS

Dans une première partie, on présente les opérateurs de Boltzmann et de Landau issus de la théorie cinétique des gaz et de la théorie des plasmas collisionnels. On explique la structure entropique de ces opérateurs (théorème H de Boltzmann), et on introduit les conjectures de Cercignani, et leur preuve dans certains cas particuliers (travaux de Toscani-Villani, LD-Villani et Villani). Ces conjectures peuvent être vues comme une version quantitative du théorème H de Boltzmann, et permettent de donner des estimations explicites de convergence vers l'équilibre thermodynamique pour un gaz homogène. La […]

Elimination of imaginaries for differentially closed fields of finite characteristic

Sophie Germain salle 1016

All fields under discussion here are assumed to have finite characteristic p. This talk might be seen as a sequel to my survey talk at Françoise Delon's conference in June 2016, although it will not assume familiarity with this talk.Of interest here are two complete theories, namely differentially closed fields (DCF) and separably closed fields (inf-SCF) with infinite degree of imperfection. These theories are related. For example, the underlying field of a model of DCF is a model of inf-SCF, and the constant field is also a model of inf-SCF. […]

Rational points on families of curves

ENS Salle W

The TAC (torsion anomalous conjecture) states that for an irreducible variety V embedded transversaly in an abelian variety A there are only finitely many maximal V-torsion anomalous varieties. It is well know that the TAC implies the Mordell-Lang conjecture. S. Checcole, F. Veneziano and myself were trying to prove some new cases of the TAC. In this process we realised that some methods could be made not only effective but even explicit. So we analysed the implication of this explicit methods on the Mordell Conjeture. Namely: can we make the […]

Quasianalytic Ilyashenko algebras

ENS Salle W

In 1923, Dulac published a proof of the claim that every real analytic vector field on the plane has only finitely many limit cycles (now known as Dulac's Problem). In the mid-1990s, Ilyashenko completed Dulac's proof

Satellites of spherical subgroups and Poincaré polynomials

ENS Salle W

Let G be a connected reductive group over C. One can associate with every spherical homogeneous space G/H its lattice of weights X^*(G/H) and a subset S of M of linearly independent primitive lattice vectors which are called the spherical roots. For any subset I of S we define, up to conjugation, a spherical subgroup H_I in G such that dim H_I = dim H and X^*(G/H_I) = X^*(G/H). We call the subgroups H_I the satellites of the spherical subgroup H. Our interest in satellites H_I is motivated by the […]

Kappa-bounded exponential groups and exponential-logarithmic power series fields without log-atomic elements

Sophie Germain salle 1016

A divisible ordered abelian group is an exponential group if its rank as an ordered set is isomorphic to its negative cone. Exponential groups appear as the value groups of ordered exponential fields, and were studied in . In we gave an explicit construction of exponential groups as Hahn groups of series with support bounded in cardinality by an uncountable regular cardinal kappa. An exp-log series s is said to be log atomic if the nth-iterate of log(s) is a monomial for all n in N. In this talk I […]

Trois exposés en théorie des groupes

ENS Toits du DMA salle W

14.00-14.45 Camille Horbez (Orsay): Boundary amenability of Out(Fn)15.00-15.45 Romain Tessera (Orsay): Poincaré profile in Hyperbolic groups15.45-16.15 pause café16.15-17.00 Yash Lodha (EPFL Lausanne): Nonamenable groups of piecewise projective homeomorphisms

Combinatoire des polyèdres convexes

ENS (amphithéâtre Galois sous la bibliothèque de mathématique)

Cet exposé est une invitation à réfléchir aux formes des polyèdres convexes et compacts de dimension finie quelconque. J’expliquerai que lorsque le polyèdre est générique du point de vue de ses faces de dimension maximale, cette forme peut être reconstituée à partir du graphe formé par les sommets et les arêtes du polyèdre. Puis j’expliquerai que lorsque le polyèdre est générique du point de vue des sommets, cela n’est plus possible. Enfin, je parlerai de la caractérisation des suites de nombres de sommets des polyèdres génériques. La situation pour les polyèdres non-génériques reste ouverte.

NSOP_1, Kim-independence, and simplicity at a generic scale

Sophie Germain salle 1016

The class of NSOP_1 theories properly contains the simple theories and is contained in the class of theories without the tree property of the first kind. We will describe a notion of independence called Kim-independence, which corresponds to non-forking independence 'at a generic scale.' In an NSOP_1 theory, Kim-independence is symmetric and satisfies a version of Kim's lemma and the independence theorem. Moreover, these properties of Kim-independence individually characterize NSOP_1 theories. We will talk about what Kim-independence looks like in several concrete examples: parametrized equivalence relations, Frobenius fields, and vector […]

Adapting to unknown noise level in super-resolution

ENS Salle W

We study sparse spikes deconvolution over the space of complex-valued measures when the input measure is a finite sum of Dirac masses. We introduce a new procedure to handle the spike deconvolution when the noise level is unknown. Prediction and localization results will be presented for this approach. An insight on the probabilistic tools used in the proofs could be briefly given as well.

Covariant LEAst-Square Re-fitting for image restoration

Salle W (ENS)

We propose a new framework to remove parts of the systematic errors affecting popular restoration algorithms, with a special focus for image processing tasks. Generalizing ideas that emerged for l1 regularization, we develop an approach re-fitting the results of standard methods towards the input data. Total variation regularizations and non-local means are special cases of interest. We identify important covariant information that should be preserved by the re-fitting method, and emphasize the importance of preserving the Jacobian (w.r.t. the observed signal) of the original estimator. Then, we provide an approach […]