Soit G un groupe hyperbolique sans torsion, soit S une partie génératrice finie de G, et soit f un automorphisme de G. Nous cherchons à comprendre les taux de croissance possibles pour la longueur d’un élément g du groupe G (écrit comme un mot en les générateurs dans S) sous l’itération de f. Le cas où G est un groupe de surface ou un groupe libre est compris grâce à des résultats de Thurston et Bestvina-Handel. Nous montrons qu’en général, il n’y a qu’un nombre fini de taux de croissance exponentiels possibles lorsque l’élément g parcourt G. Par ailleurs, nous montrons la dichotomie suivante : tout élément a une croissance qui est soit exponentielle, soit polynomiale. Ceci est un travail en commun avec Rémi Coulon, Arnaud Hilion et Gilbert Levitt.

In vitro branched actin networks have been shown to generate a pushing force. In vivo these networks have been identified at different subcellular locations to drive membrane protrusion and remodeling during intracellular traffic. Here we will discuss the integration of positive and negative regulation of membrane protrusions and how it impacts the essential parameters of cell migration. Surprisingly we found that the signaling pathway that generates branched actin at membrane protrusions, Rac-WAVE-Arpin, also controls cell cycle progression.

In fact, this signaling pathway has all the expected properties of a cell cycle checkpoint. It is required in normal cells but this requirement is lost in cancer cells. The branched actin network of membrane protrusions integrates growth factor stimulation with mechanotransduction of cell adhesion to instruct the cell that its environment is permissive for migration and cell cycle progression.

A classical problem in mathematics is the determination of the minimal surface that spans a given contour, which can be realized in the laboratory as a soap film supported by a wire frame. In the early 1940s Richard Courant pointed out nontrivial situations in which a small deformation of certain frames can render unstable the supported surface, leading by a rapid dynamical process to a new minimal surface. For example, a soap film Möbius strip can transition to a disc. Despite the enormous body of work on the mathematics of minimal surfaces themselves, the understanding of these dynamical problems is at a very early stage. In this talk I will summarize our recent experimental and theoretical work on problems of this type, in which a combination of high-speed imaging and stability theory has revealed new insights. (Work done in collaboration with A.I. Pesci, H.K. Moffatt, T. Machon and G.P. Alexander)

Séminaire de Relativité Générale Mathématique

 

In theoretical physics, it is often conjectured that a correspondence exists between the gravitational dynamics of asymptotically Anti-de Sitter (AdS) spacetimes and a conformal field theory of their boundaries. In the context of classical relativity, one can attempt to rigorously formulate such a correspondence statement as a unique continuation problem for PDEs: Is an asymptotically AdS solution of the Einstein equations uniquely determined by its data on its conformal boundary at infinity? In this presentation, we establish a key step: we prove such a unique continuation result for wave equations on fixed asymptotically AdS spacetimes. In particular, we highlight the analytic and geometric features of AdS spacetime which enable this uniqueness result, as well as obstacles preventing such a result from holding in other cases. If time permits, we will also discuss some applications of this result toward symmetry extension and rigidity theorems.

Les foncteurs exponentiels (gradués commutatifs) apparaissent naturellement dans un certain nombre de calculs homologiques (homologie des groupes, des foncteurs…). Dans cet exposé, nous donnerons quelques résultats de structure des foncteurs exponentiels et des applications à des calculs concrets.

Soient V=(Z/2Z)n et X un V-CW complexe fini. On construit deux complexes dans la catégorie H*V-U, le premier "algébrique" basé sur la structure de H*V-module de la cohomologie équivariante H*_VX et le second "topologique" utilisant l'action du groupe V sur X. Sous certaines conditions, on montre que ces deux complexes sont acycliques et équivalents.

We compare height functions for (1) points of an algebraic variety over a number field, (2) motives over a number field, (3) variations of Hodge structure with log degeneration on a projective smooth curve over the complex number field, (4) horizontal maps from the complex plane C to a toroidal partial compactification of the period domain. Usual Nevanlinna theory uses height functions for (5) holomorphic maps f from C to a compactification of an algebraic variety V and considers how often the values of f lie outside V. Vojta compares (1) and (5). In (4), V is replaced by a period domain. The comparisons of (1)–(4) provide many new questions to study.

I will make a quick review of old and new results concerning the Lorentz gas; discuss new directions in which I’d like to proceed (e.g. non periodic obstacles, interacting particles, …) and some (very) partial results toward such directions.

The two-dimensional one-component Coulomb plasma is an ensemble of identical charges in the plane repelling each other via the logarithmic Coulomb potential and confined by an external potential. I will discuss results that show that the fluctuations of linear statistics of many particles are of order 1 and given by a Gaussian free field, at any temperature. This is joint work with P. Bourgade, M. Nikula, and H.-T. Yau.

Totally positive matrices are characterized by having all their minors positive. They appear in various areas of physics and mathematics, including oscillations in mechanical systems, quantum groups, and algebraic geometry. It has been known since the work of Fomin that the two-point correlations functions of the two-dimensional Gaussian free field satisfy total positivity. I will present an analogous result for the correlations of the planar Ising model. The idea is to prove that determinants of such correlations have interpretations in terms of probabilities of events in the random current model. A natural open question is to identify all planar totally positive spin systems.

We study the gradient field models with uniformly convex potential (also known as the Ginzburg-Landau field) in two dimension. These log-correlated non-Gaussian random fields arise in different branches of statistical mechanics. Existing results were mainly focused on the CLT for the linear functionals. In this talk I will describe some recent progress on the global maximum and local CLT for the field, thus confirming they are in the Gaussian universality class in a very strong sense. The proof uses a random walk representation (a la Helffer-Sjostrand) and an approximate harmonic coupling (by J. Miller).