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  • 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: // / 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


    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


    • 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 Erdős-Szemerédi 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 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

Conference - Families of algebraic dynamical systems

Rennes, from June 12th to June 16th

Organization board: Serge Cantat, Christophe Dupont

Scientific board: Matthew Baker, Eric Bedford, Serge Cantat, Christophe Dupont, Mattias Jonsson

Mini-courses :

  • Bertrand Deroin (Ecole Normale Supérieure, Paris) Holomorphic families of representations in SL(2,C)

We will survey some aspect of the theory of holomorphic families representations in SL(2,C):
1. Sullivan's stability theory
2. Bifurcation currents
3. Harmonic measures of complex projective structures

  • Charles Favre (Ecole Polytechnique, Palaiseau) Degeneration of rational maps of the Riemann sphere

We shall describe how one can control the dynamics of a meromorphic family of rational maps of the Riemann sphere parameterized by the punctured unit disk as one approaches the puncture. Our analysis is based in a crucial way on the interplay between complex and non-archimedean dynamics. We shall also review how this control can be combined with technics from arithmetic geometry to the description of the special curves in the parameter space that contain infinitely many post-critically finite maps.

  • Laura de Marco (Northwestern University, Chicago) Rational maps, elliptic curves, and heights

We will study the geometry and arithmetic of families of rational maps and families of elliptic curves. The focus will be on "canonical height functions", introduced by Tate and Neron around 1960 in the setting of abelian varieties and further developed by Call and Silverman (1993) for algebraic dynamical systems. My aim is to present recent results -- both in the setting of elliptic curves and of rational maps -- and to present open questions inspired by the connections between holomorphic dynamics and arithmetic geometry.

Talks :

  • François Berteloot (Toulouse): Bifurcations within holomorphic families of endomorphisms of P^k
  • Simon Brandhorst (Hannover): On the dynamical spectrum of projective K3 surfaces
  • Romain Dujardin (Université Paris 6): Degenerations of SL(2,C) representations and Lyapunov exponents
  • Alexander Gamburd (City University of New-York): Markov triples and strong approximation
  • Thomas Gauthier (Université de Picardie Jules Verne, Amiens): The support of the bifurcation measure has positive volume
  • Martin Hils (Paris): Model theory of compact complex manifolds with an automorphism
  • Sarah Koch (Ann Harbor): Irreducibility of curves in parameter space: cubic polynomials vs. quadratic rational maps
  • Holly Krieger (Cambridge University): Reduction of dynatomic curves
  • Juan Rivera-Letelier (Rochester): Hecke and Linnik
  • Thomas Scanlon (Berkeley University): Applications of characterizations of skew-invariant varieties
  • Tom Tucker (Rochester): Towards a finite index conjecture for iterated Galois groups
  • Junyi Xie (Université de Rennes 1): Invariant pencils for polynomial selfmaps of the affine plane

Abstracts :

François Berteloot : Bifurcations within holomorphic families of endomorphisms of P^k.

Simon Brandhorst : On the dynamical spectrum of projective K3 surfaces.

The dynamical degree of a surface automorphism is a Salem number, that is, an algebraic integer lambda>1 which is conjugate to 1/\lamda and all whose other conjugates lie on the unit circle. We prove that for each Salem number lambda of degree at most 20, there is a power lambda^n, n in N, which is the dynamical degree of an automorphism of some projective K3 surface.

Romain Dujardin : Degenerations of SL(2,C) representations and Lyapunov exponents

The talk is a report of work in progress with Bertrand Deroin and Charles Favre. Let G be a finitely generated group endowed with some probability measure mu and (rho_lambda) be an algebraic family of representations of G into SL(2,C), diverging in the representation space as lambda converges to infinity. Using non-Archimedean techniques, we study the asymptotics of the random product of matrices induced by rho_lambda(G,mu) as lambda converges to infinity. In particular we can describe the growth rate of the Lyapunov exponent in terms of non-Archimedean data.

Alexander Gamburd : Markov triples and strong approximation.

Thomas Gauthier : The support of the bifurcation measure has positive volume.

The moduli space M_d of degree d>=2 rational maps can naturally be endowed with a measure mu_bif detecting maximal bifurcations, called the bifurcation measure. We prove that the support of the bifurcation measure mu_bif has positive Lebesgue measure. To do so, we establish a general criterion for the conjugacy class of a rational map to belong to the support of mu_bif and we exhibit a "large" set of Collet-Eckmann rational maps which satisfy that criterion. As a consequence, we get a set of Collet-Eckmann rational maps of positive Lebesgue measure which are approximated by hyperbolic rational maps. This is a joint work with Matthieu Astorg, Nicolae Mihalache and Gabriel Vigny.

Martin Hils : Model theory of compact complex manifolds with an automorphism

One may develop the model theory of compact complex manifolds (CCM) with a generic automorphism in rather close analogy to what has been done for existentially closed difference fields, in important work by Chatzidakis and Hrushovski, among others. The corresponding first order theory CCMA provides a model-theoretic framework for the study of meromorphic dynamical systems. In the talk, I will present some results from 'geometric model theory' which hold in CCMA (e.g. the Zilber trichotomy for 'finite-dimensional' types). This is joint work with Martin Bays and Rahim Moosa.

Sarah Koch : Irreducibility of curves in parameter space: cubic polynomials vs. quadratic rational maps

Living inside the space of monic centered cubic polynomials, are the curves S_n, which consist of all polynomials f which possess a superattracting cycle of period n. Recently, Arfeux and Kiwi announced a proof that S_n is irreducible for all n>=1. In this talk, we consider the analogous curves which live in the moduli space of quadratic rational maps. It is currently unknown if these curves are irreducible. We discuss some unexpected challenges that arise in the quadratic rational map setting which are absent in the cubic polynomial setting. This talk is based on joint work with E. Hironaka.

Holly Krieger : Reduction of dynatomic curves

Dynatomic curves parametrize n-periodic orbits of a one-parameter family of polynomial dynamical systems. These curves lack the structure of their arithmetic-geometric analogues (modular curves of level n) but can be studied dynamically. Morton and Silverman conjectured a dynamical analogue of the uniform boundedness conjecture (theorems of Mazur, Merel), asserting uniform bounds for the number of rational periodic points for such a family. I will discuss recent work towards the function field version of their conjecture, including results on the reduction mod p of dynatomic curves for the quadratic polynomial family z^2+c.

Juan Rivera-Letelier (Rochester): Hecke and Linnik

I will discuss the equidistribution of Hecke operators of p-adic elliptic curves. The most difficult case, of supersingular elliptic curves, is analyzed using Lubin-Katz theory of the canonical subgroups, and the period map of Serre-Tate's theory on the deformation of fomal groups. The key ingredient is a version of Linnik's equidistribution theorem for a certain p-adic quaternion algebra. This is a joint work with Sebastian Herrero and Ricardo Menares.

Thomas Scanlon : Applications of characterizations of skew-invariant varieties

In work with Medvedev, I classified the skew-invariant subvarieties of so-called split polynomial dynamical systems. Here, a split polynomial dynamical system is one of the form F: A^n to A^n given in coordinates as (x_1,...,x_n) -> (f_1(x_1), f_2(x_2),..., f_n(x_n)) where each f_i is a polynomial in one variable. The "skew" in "skew-invariant" means that we work over a field K equipped with an endomorphism sigma : K -> K. A subvariety V of A^n is skew-invariant if F maps V to V^sigma, the transform of V under sigma. In most applications of our theorem to date, only the case that sigma is the identity is used and the resulting classification of the invariant varieties may be obtained from methods of complex dynamics, as shown by Pakovich. In this lecture, I will speak about two applications which make essential use of the generalization to skew-invariance: a theorem proven jointly with Medvedev and Nguyen that Mahler functions of polynomial type with respect to multiplicatively independent exponents are algebraically independent and a project with Medvedev to extend our classification of (skew-)invariant varieties to what we call triangular dynamical systems (though what have been called skew-products in the literature): algebraic dynamical systems of the form F : A^n to A^n given in coordinates as (x_1,...,x_n) -> (f_1(x_1),f_2(x_1,x_2),...,f_n(x_1,...,x_n)) where f_i is a polynomial in the variables x_1,..., x_i.

Tom Tucker : Towards a finite index conjecture for iterated Galois groups

Let f be a polynomial over a global field. Let G denote the inverse limits of the Galois groups of f^n, where f^n denotes n-th iterate of f. Boston and Jones have suggested that under reasonable hypotheses, one might hope that G has finite index in the full group of automorphisms on an infinite tree corresponding to roots of iterates f^n when f is quadratic. We will show that their conjecture is true over function fields of characteristic 0, and that it would be a consequence of well-known diophantine conjectures over number fields. We will also treat the case of cubic polynomials, where less is known.

Junyi Xie : Invariant pencils for polynomial selfmaps of the affine plane

With Jonsson and Wulcan, we classify polynomial selfmaps f of the affine plane of that preserve an irreducible pencil of curves at infinity. More generally, we study a more general classification problem, where the invariant pencil is replaced by more general numerical data at infinity.




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