• Conference - Young researcher meeting in dynamics and geometry
Sep 6, 2017 to Sep 8, 2017

Rennes, from September 6th to September 8th

Organization board: Françoise Dal'Bo, Frédéric Paulin, Barbara Schapira, Damien Thomine

Since its creation the Platon network (GDR National Center for Scientific Research n°3341 http: // costia.free.fr / platon/) leads actions towards young researchers in ergodic geometry. The recurrent young researcher meeting is one of the highlights of the year. The goal is to allow about ten PhD students or recent doctors to expose their work and promotes discussions between young and senior researchers. The "Young researcher meeting in dynamics and geometry" follows the spirit of these recurring meetings with an international dimension brought in particular by Swiss and Senegalese networks.

See also here

TALKS

Alexander Adam (UPMC) Resonances for Anosov diffeomorphism

Kamel Belarif (Université de Bretagne Occidentale) Genericity of weak mixing in negative curvature

Adrien Boulanger (UPMC) Cascades in affine interval exchanges

Filippo Cerocchi (Max Planck Institute for Mathematics, Bonn) Rigidity and finiteness for compact 3-manifolds with bounded entropy

Maria Cumplido Cabello (Université de Rennes 1) Loxodromic actions of Artin-Tits groups

Nguyen-Bac Dang (Ecole Polytechnique) Degrees of iterates of rational maps

Laurent Dufloux (Oulu University) Hausdorff dimension of limit sets at the boundary of the complex hyperbolic plane

Mikolaj Fraczyk (Université Paris-Sud) Mod p homology growth of locally symmetric spaces

Weikun He (Université Paris-Sud) Sum-product estimates and equidistribution of toral automorphisms

Cyril Lacoste (Université de Rennes 1) Dimension rigidity of lattices in semisimple Lie groups

Erika Pieroni (Università di Roma, Sapienza) Minimal Entropy of 3-manifolds

Fanni M. Selley (Budapest University of Technology) Ergodicity breaking in mean-field coupled map systems

Nasab Yassine (Université de Bretagne Occidentale) Quantitative recurrence of one-dimensional dynamical systems preserving an infinite measure

ABSTRACTS

• Alexander Adam Resonances for Anosov diffeomorphism

The deterministic chaotic behavior of an invertible map T is appropriately described by the existence of expanding and contracting directions of the differential of T. A special class of such maps consist in Anosov diffeomorphisms. Every 2-by-2 hyperbolic matrix M with integer entries induces such a diffeomorphism on the 2-torus. For all pairs of real-analytic functions on the 2-torus, one defines a correlation function for T which captures the asymptotic independence of such a pair under the evolution T^n as n tends to infinity. What is the rate of convergence of the correlation as n tends to infinity, for instance what is its decay rate? The resonances for T are the poles of the Z-transform of the meromorphic continued correlation function. The decay rate is well-understood if T=M. There are no non-trivial resonances of M. In this talk, I consider small real-analytic perturbations T of M where at least one non-trivial resonance of T appears. This affects the decay rate of the correlation.

• Kamel Belarif Genericity of weak mixing in negative curvature

Let M be a manifold with pinched negative sectional curvature. We show that, when M is geometrically finite and the geodesic flow on T^1M is topologically mixing, the set of mixing invariant measures is dense in the set P(T^1M) of invariant probability measures. This implies that the set of weak-mixing measures which are invariant by the geodesic flow is a dense G-delta subset of P(T^1M). We also show how to extend these results to geometrically infinite manifolds with cusps or with constant negative curvature.

• Adrien Boulanger Cascades in affine interval exchanges

Avec un échange d'intervalle affine donné vient naturellement une famille de telles dynamiques indexées par le cercle. En effet, la pré-composition par une rotation de l'application initiale définit un autre échange d'intervalle affine. On étudiera cette famille de dynamiques dans un cas particulier à travers la géométrie de la surface affine associée et son groupe de transformation affine.

An affine interval exchange (AIE) is a piecewise affine map from the circle to itself. Such a map defines a dynamical systems over the circle by iterating it. With an AIE comes naturally a family of AIE indexed by the circle: they are defined by pre-composing the initial AIE by a rotation. The presentation will focus on the study of possible dynamical behaviors of such a family of AIE through a peculiar example.

• Filippo Cerocchi Rigidity and finiteness for compact 3-manifolds with bounded entropy

We present some local topological rigidity results for the set S of non-geometric, compact -- with possibly empty boundary and no spherical boundary components --, orientable Riemannian 3-manifolds having torsionfree fundamental group, with bounded entropy and diameter. By "local", we mean that we consider S endowed with the Gromov-Hausdorff-topology. We shall provide examples to show the necessity of the assumptions and discuss some open problems. Moreover, we shall give a proof of the finiteness of the homeomorphism types of the manifolds in S. These are joint works with A. Sambusetti (Rome, Sapienza).

• Maria Cumplido Cabello Loxodromic actions of Artin-Tits groups

Artin-Tits groups act on a certain delta-hyperbolic complex, called the additional length complex". For an element of the group, acting loxodromically on this complex is a property analogous to the property of being pseudo-Anosov for elements of mapping class groups. A well-known conjecture about mapping class groups claims that "most elements" of the mapping class group of a surface are pseudo-Anosov. In fact, we can prove that a positive proportion is pseudo-Anosov.

By analogy, we conjecture that most'' elements of Artin-Tits groups act loxodromically. More precisely, in the Cayley graph of a subgroup G of an Artin-Tits group, the proportion of loxodromically acting elements in a ball of large radius should tend to one as the radius tends to infinity. We will give a condition guaranteeing that this proportion stays away from zero. This condition is satisfied e.g. for Artin-Tits groups of spherical type, their pure subgroups and some of their commutator subgroups.

• N'Guyen-Bac Dang Degrees of iterates of rational maps

In this talk, I will explain what is a rational map, how to define its k-degrees, and I will study the k-degrees of its iterates. I will explain how the study of the growth of these sequences of numbers helps in understanding the dynamics of these maps.

• Laurent Dufloux Hausdorff dimension of limit sets at the boundary of complex hyperbolic planes

Consider the standard contact structure on the 3-sphere. The associated subriemannian metric has dimension 4. The Gromov comparison problem asks about how the Hausdorff dimension with respect to this subriemannian metric is related tothe Hausdorff dimension with respect to the usual (Riemannian) metric. We will look at this problem in the case of limit sets of discrete groups of complex hyperbolic isometries.

• Mikolaj Fraczyk Mod p homology growth of locally symmetric spaces

I will talk about the growth of the dimension of mod-p homology groups of locally symmetric spaces. Let G be a higher rank Lie group and X its symmetric space and let L be a lattice in G. Results on the rank gradient by Abert, Gelander and Nikolov imply that if L is right angled then for every sequence of subgroups (L_n) of L, the dimensions of the homology groups H_1(X/L_n,Z/pZ) grow sublinearly in the volume of X/L_n. In the special case p=2, I showed that the same statement holds for any sequence of lattices L_n with volume escaping to infinity (even if they are pairwise non-commensurable).

• Weikun He Sum-product estimates and equidistribution of toral automorphisms

Bourgain's sum-product theorem is a metric version of ErdÅ‘s-SzemerÃ©di sum-product theorem. It asserts that a typical set of real numbers grows fast under addition and multiplication. We will present a generalisation of Bourgain's theorem to matrix algebras and discuss how it is motivated by a ergodic problem, namely, quantitative equidistributions of orbits on the d-dimensional torus under sub-semigroups of SL(d,Z).

• Cyril Lacoste Dimension rigidity of lattices in semisimple Lie groups

We study actions of discrete groups on classifying spaces (or classifying spaces for proper actions). For instance the hyperbolic plane is a classifying space for proper actions of the group PSL(2,Z) (but not of minimal dimension). Such spaces can be used to compute the cohomology of the group, so we want them to have the lowest possible dimension. This leads us to the definitons of the (proper) geometric dimension and the (virtual) cohomological dimension. These two dimensions are not always equal, we will see it is the case for a lattice in the group of isometries G of a symmetric space of non-compact type without Euclidean factors (such a group is a semisimple Lie group but not necessarily connected). This result has an important consequence called "dimension rigidity", that is, the two dimensions are still equal for a group commensurable to a lattice of G.

• Erika Pieroni Minimal Entropy of 3-manifolds

We present the solution of the minimal entropy problem for non-geometric, closed, orientable 3-manifolds (that is, those manifolds which do not admit a com- plete metric locally isometric to one of the eight 3-dimensional model geometries). Together with the results of Besson-Courtois-Gallot for locally symmetric spaces and the work of Soma, Gromov et.al. on the simplicial volume of 3-manifolds and its relation with entropy, this gives a complete picture of the minimal entropy prob- lem for all closed, orientable 3-manifolds. Our work strongly builds on Souto's PhD work (unpublished), filling some gaps in the proof and completing the picture in the case of non-prime manifolds. In detail, we show that the minimal entropy is ad- ditive with respect to the prime decomposition and that for an irreducible manifold X it coincides with the sum of the volume entropies of all the JSJ components of hyperbolic type, each endowed with its complete, hyperbolic metric of nite volume. For the lower bound of MinEnt(X), we adapt Besson-Courtois-Gallot's barycenter method following Souto's ideas; then, we show how this lower bound is realized by producing a sequence of Riemannian metrics gk on X whose volume-entropies tend to

• Fanni M. SelleyErgodicity breaking in mean-field coupled map systems

Coupled map systems are simple models of a finite or infinite network of interacting units. The dynamics of the compound system is given by the composition of the (typically chaotic) individual dynamics and a coupling map representing the characteristics of the interaction. The coupling map usually includes a parameter s in [0,1], representing the strength of interaction. The main interest in such models lies in the emergence of bifurcations when s is varied. We first introduce our results for small finite systems. Then we initiate a new point of view which focuses on the evolution of distributions and allows to incorporate the investigation of a continuum of sites.

• Nasab Yassine Quantitative recurrence of one-dimensional dynamical systems preserving an infinite measure

We are interested in the asymptotic behaviour of the first return time of the orbits of a dynamical system into a small neighbourhood of their starting points. We study this quantity in the context of dynamical systems preserving an infinite measure. More precisely, we consider the case of Z-extensions of subshifts of finite type. We also consider a toy probabilistic model in order to enlighten the strategy of our proofs.

• Conference - Geometric Analysis at Roscoff
Oct 9, 2017 to Oct 13, 2017

Roscoff, from October 9th to October 13th

Organization board: Paul Baird, Gilles Carron, Ali Fardoun, Carl Tipler

Scientific board: Gérard Besson (CNRS, Institut Fourier), Olivier Biquard (ENS Paris), Ahmad El Soufi (Univ. Tours)

Geometric Analysis is the application and development of PDE tools and technics in Riemannian geometry, it is also a fundamental tool in mathematical physics. Recently, important conjectures has been solved: Poincaré's conjecture, Willmore's conjecture, Lawson's conjecture, Yau-Tian-Donaldson's conjecture and a lot of new tools has been introduced and developed : optimal transport, weak formulation of Ricci curvature, Geometric measure theory. This conference will be an opportunity for specialists from theses different areas to meet and exchange ideas, questions and knowledge.

• Conference - Lebesgue PHD meeting 2017
Oct 16, 2017 to Oct 18, 2017

Rennes, from October 16th to October 18th

Organization board: Grégory Boil, Valentin Doli, Caroline Robet, Jérôme Spielmann

Scientific board: Solène Bulteau, Clément Rouffort, Nasab Yassine

Depuis trois ans, le Centre Henri Lebesgue soutient les Rencontres doctorales Lebesgue, initiative des doctorants du Labex. Il s'agit de trois journées de conférences durant lesquelles la parole est donnée à des doctorants de tout horizon géographique et mathématique. L'objectif est ainsi de présenter un panel le plus large possible de la recherche mathématique actuelle telle qu'elle est vue et vécue par les doctorants, mais pas seulement... Lors de ces rencontres, trois chercheurs, appelés 'parrains' de l'évènement, sont invités à exposer et ainsi à partager leur expérience personnelle de la recherche d'aujourd'hui. Cette année, les rencontres sont parrainées par :

Jean-Marc Bardet (SAMM, Université Paris 1);

Jasmin Raissy (Institut mathématique de Toulouse, Université Paul Sabatier);

Gabriel Rivière (Laboratoire Paul Painlevé, Université de Lille 1).

Bien que principalement destinée aux doctorants, cette conférence se veut également accessible aux étudiants de M2 désireux d’avoir un aperçu des travaux auxquels une thèse en mathématiques peut mener. Pour les doctorants désireux d'exposer leurs travaux de recherche, il est également possible de déposer une proposition sur l'onglet 'Proposer un Exposé'. Dans un souci d'organisation, merci de vous inscrire à l'évènement même si vous proposez un exposé.

• Complex dynamics and quasi-conformal geometry
Oct 23, 2017 to Oct 25, 2017

Our colleague Tan Lei passed away in April 2016. A conference will be held from 23/10/2017 to 25/10/2017 at the University of Angers to honour her memory.

### Scientific Committee

Etienne Ghys (ENS Lyon)
John Milnor (Stony Brook)
Mitsuhiro Shishikura (Kyoto).

### Organizing Committee

Mohammed El Amrani (Angers)
Michel Granger(Angers)
Jean-Jacques Loeb(Angers)
Laurent Meersseman(Angers)
Pascale Roesch(Toulouse).

### Provisional list of speakers

Xavier Buff, Arnaud Cheritat, Nuria Fagella (to be confirmed), Cui Guizhen,Peter Haissinski, John Hamal Hubbard (to be confirmed), Carsten lunde Petersen, Kevin Pilgrim, Mary Rees, Pascale Roesh, Hans Henrik Rugh, Dylan Thurston, Mitsu Shishikura, Giulio Tiozzo.

More information : page

The registration process is already open.

• School - Masterclass 2017
Dec 19, 2017 to Dec 21, 2017

Angers, from December 19th to December 21st

Organization board: Etienne Mann

Nous organisons un masterclass à Angers du 19 au 21 décembre 2017.

• Workshop - Numerical methods for algebraic curves
Feb 19, 2018 to Feb 23, 2018

Rennes, from February 19th to February 23rd

Organization board: Christophe Ritzenthaler

• Conference - Maths and Business Days
Apr 12, 2018 to Apr 13, 2018

Vannes, from April 12th to April 13th

Organization board: Christophe Berthon, Eric Darrigrand, Emmanuel Frénod, Fabrice Mahé

• School - Fundamentals and practice of finite elements
Apr 16, 2018 to Apr 20, 2018

Roscoff, from April 16th to April 20th

Organization board: Monique Dauge, Ilaria Peruggia

# 5 minutes Lebesgue

en cours d'évaluation

## Axel Rogue, P et NP

On explique ce que signifie la fameuse question P=NP qui vaut 1 million de dollars.

en cours d'évaluation

## Didier Robert, Le côté quantique du chaos, l'exemple du billard

Dans le monde quantique jouer sur un billard en forme de stade plonge le joueur dans une situation imprévisible.

en cours d'évaluation

## Niccolo Torri, Le problème de numérotation des sièges dans le train

Un peu de combinatoire autour d'un problème qui nous touche lors d'un grand voyage en train.

en cours d'évaluation

## Jean-Philippe Nicolas, Trous noirs et superradiance

Où l'on explique comment extraire de l'énergie des trous-noirs pour fabriquer des centrales ou des bombes...

en cours d'évaluation

## Éric Darrigrand, Quand Saturne échappe aux méthodes multipôles rapides

La simulation du système solaire est un problème académique qui permet l'analyse du comportement des méthodes numériques.

en cours d'évaluation

## Nicolas Raymond, Oscillations dans un champ magnétique

Nous expliquons mathématiquement le mouvement d'un électron dans un champ magnétique non uniforme.

en cours d'évaluation

## Baptiste Chantraine, Le théorème de Girard pour les triangles sphériques

Sur une sphère, la somme des angles d'un triangle est reliée à son aire.

en cours d'évaluation

## Basile Pillet, Cohomologie des figures impossibles

Le triangle de Penrose est un objet imaginaire qu'on peut dessiner mais pas construire en 3D.

en cours d'évaluation

## Bachir Bekka, Le théorème de Mazur-Ulam

Le théorème de Mazur-Ulam établit un lien entre la structure métrique et la structure affine d'un espace vectoriel normé.

en cours d'évaluation

## Barbara Schapira, Mathématiques électorales

Y a-t-il une façon de voter meilleure que les autres ?

en cours d'évaluation

## Marc Raynaud, La machine de Turing

Du concept à la réalisation avec un aperçu de toutes les possibilités algorithmiques d'un simple système électro-mécanique.

en cours d'évaluation

## Miguel Rodrigues, Comment mesurer la taille d'une fonction ?

Que souhaite-t-on mesurer ? Amplitude, support, fréquence ? Comment le quantifier ?

en cours d'évaluation

## Jean-François Coulombel, Sur la stabilité des suites géométriques

On aborde quelques aspects de l'étude des suites géométriques dans des algèbres de Banach

en cours d'évaluation

## Xavier Caruso, Vincent Duchêne, Deux jeux mathématiques

Jouer avec les nombres ou avec les figures géométriques.

en cours d'évaluation

## Olivier Pierre, Comment illustrer des calculs de sommes d'entiers ?

On illustre géométriquement des formules bien connues de sommes d'entiers.

en cours d'évaluation

## San Vũ Ngọc, Huygens et le pendule magique

Quelle est donc cette mystérieuse courbe qui permet à Huygens en 1657 de construire des horloges à balancier très précises ?

en cours d'évaluation

## Jérémy Le Borgne, Peut-on ne pas inverser la courbe du chômage ?

Lorsqu'on cherche à inverser la courbe du chômage, on se contente parfois d'une croissance moins rapide...

en cours d'évaluation

## Pierre Joray, Définir la négation

d'après Tarski, une nouvelle façon de voir la logique...

étudiant

## Guillem Cazassus, Voleurs de colliers et sandwichs au jambon

Nous expliquerons comment un résultat de topologie peut être utilisé à des fins criminelles.

enseignant

## Jean-Pierre Escofier, Autour du théorème de Lagrange pour les groupes finis

Le théorème de Lagrange énonce que l’ordre d’un sous-groupe d’un groupe fini divise l’ordre de ce groupe...

enseignant

## Joe Viola, Ressort, mécanique quantique et incertitude

Nous proposons une petite introduction à la mécanique quantique en étudiant d'abord le modèle classique du ressort.

étudiant

## Felix Ulmer, La spirale de Théodore de Cyrène

Platon attribue à Théodore de Cyrène la preuve de l’irrationalité des racines de 3, 5, 6, 7, 8, 10, 11, 12, 13, 14 15 et 17.

étudiant

## Bernard Delyon, Les dés du diable

Un jeu de dés un peu spécial pour lequel le deuxième joueur a une stratégie gagnante en moyenne

grand public

## Thomas Wallez, Et si les criminels devaient aussi se méfier des mathématiciens ?

Une enquête policière fictive qui nous mène vers la théorie des graphes.

scientifique

## Frédéric Hélein, Les problématiques de l'édition scientifique

Comment fonctionne la publication des travaux scientifique ? Quel est le rôle des mathématiciens, des bibliothèques et des éditeurs ?

étudiant

## Vincent Guirardel, Le problème de l'arrêt d'une machine de Turing

Peut-on savoir à l'avance si un programme d'ordinateur va s'arrêter, ou s'il va boucler à l'infini ?

grand public

## À quoi servent les mathématiques ?

Quelques chercheurs rennais tentent de répondre à cette question.

chercheur

## Xavier Saint-Raymond, Permutations dans les séries

Quelles sommes peut-on obtenir lorsque l'on s'autorise à modifier l'ordre des termes dans une série ?

enseignant

## Victoria Lebed, Battages de cartes

Combien de fois faut-il battre un jeu de cartes ?

étudiant

## Christophe Ritzenthaler, Les cent prisonniers

Une énigme mathématique basée sur de la combinatoire des permutations.