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  • Post-doc positions
    Oct 4, 2017 to Dec 1, 2017
    lebesgue_money.png Henri Lebesgue Center offers post-doc positions for researchers in mathematics.

    Description

    The Lebesgue Center and its partners, Région Bretagne & Région Pays de la Loire (DéfiMaths project), are opening applications for three post-doctoral positions in mathematics for a period of 2*12 months. The hired applicant will conduct his/her research at one of these Insitutes:

    Irmar of Rennes
    LMJL of Nantes
    LMBA of Brest and Vannes
    LAREMA of Angers

    The positions are open to all research areas present within the four Institutes. They do not include an obligation to teach.

    The net salary will be 2 100 Euros per month.

    Deadlines

    The application is open.

    Sending application: between the 4th of October 2017 and the 1st of December 2017.

    The positions are expected to begin on September 1st or October 1st, 2018.

    Eligibility

    Candidates must have completed a PhD in mathematics, or equivalent, at the date of taking office. The candidate must submit an original research project including a collaboration with one or more local researcher (s) of Irmar, LMJL,LMBA, or LAREMA.

    Applicants must complete the online form where they have to join the following documents:

    • CV describing the candidate's profile and professional experience (list of publications, research topics, activities)
    • Cover letter including the research project
    • At least two letters of recommendation, including one from the local researcher involved in the project

    For more information, please contact us at post-doctorant[at]lebesgue.fr.

    Selection

    The selection is carried by the Lebesgue Scientific Committee.

    Nominees

    2017-2018
    2016-2017
    2015-2016
    2014-2015
    2013-2014

  • 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.

  • 5 minutes Lebesgue
    Oct 24, 2017

    Les vidéos des exposés seront mises en ligne quelques jours après l'exposé. Vidéothèque

    Prochain exposé (rediffusion):

    24-10-2017:  Vincent Colin
    Comment mesurer la forme d'un espace ?

    Comment construire des espaces exotiques aux propriétés surprenantes et comment, par des expériences locales, en deviner la forme ? On se laissera guider par Henri Poincaré.

    Lieu

    Rennes

    Exposés à venir:

    07-11-2017:  Éric Hazane

    21-11-2017:  Marine Fontaine

    28-11-2017:  Roger Lewandowski

    05-12-2017:  Maria Cumplido

    19-12-2017:  Guy Casale

    06-02-2018:  Rozenn Texier-Picard

  • Séminaire Quimpériodique
    Nov 16, 2017 to Nov 17, 2017

    Ce séminaire de géométrie, complètement à l'Ouest, réunit à Quimper, trois fois l'an, pour deux journées, le jeudi et le vendredi, des géomètres venus des régions Bretagne et Pays de Loire.

    Programme

    Sorin Dumitrescu (Univ. Nice): Géométries de Cartan branchées
    Clément Fromenteau (LAREMA): Sur le champ de Teichmüller des surfaces de Hopf
    Ngoc-Phu Ha (LMBA Vannes): Invariants quantiques associés à la super-algèbre de Lie sl(2|1)
    Harold Rosenberg (IMPA): The geometry and topology of complete minimal surfaces and applications
    Caroline Vernier (LMJL): Théorèmes de recollement en géométrie Kählérienne

    Correspondants

    Guillaume Deschamps, LMBA, Brest
    Laurent Meersseman, LAREMA, Angers
    Gaël Meigniez, LMBA, Vannes
    Yann Rollin, LMJL, Nantes
    Frédéric Touzet, IRMAR, Rennes

    Historique

    Historique du séminaire

    Affiche

    Télécharger l'affiche ici

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

    QR-Code

    Angers, from December 19th to December 21st

    Organization board: Etienne Mann

    We organize a masterclass in Angers, from 19th to 21st of December, 2017. Lectures will be in the morning sessions. We will have two parallel sessions:

    • Etienne Mann : Introduction to algebraic stacks
      We will introduce the notion of fibered category, Grothendieck topology and stacks. We will illustrate these notions on simple examples. These lectures are for Master (or PhD) students who had followed a introduction to algebraic geometry that is algebraic varieties and sheaves.
    • Loic Chaumont: Application of the matrix-tree-theorem

    The organisation board will pay the housing and the lunch. The other meals are not payed by the organizers. For the travel expenses, we will do our best in the limit of our budget.

    Date limite d'inscription 15 novembre.

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

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    Rennes, from February 19th to February 23rd

    Organization board: Xavier Caruso, David Lubicz, Christophe Ritzenthaler, Marie-Françoise Roy

    This workshop will bring together researchers in complex and real algebraic geometry and applied mathematics (physics and cryptography) to discuss numerical methods and open problems on algebraic curves. There will be a number of introductory talks to each of the following topics and invited lectures from specialists: - Theoretical physics/DPE - Computations with the Jacobian - Random real topology - Complexity of computing topology of real algebraic curves - p-adic methods and applications to cryptography

  • Conference - Mathematics and Entreprises Days
    Apr 12, 2018 to Apr 13, 2018

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    Vannes, from April 12th to April 13th

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

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

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    Roscoff, from April 16th to April 20th

    Organization board: Martin Costabel, Eric Darrigrand, Monique Dauge, Yvon Lafranche

    Scientific board: Monique Dauge (Univ. Rennes 1), Ilaria Perugia (TU Wien)

Conference - Young researcher meeting in dynamics and geometry

in partnership with GDR N° 3341 Platon

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

See the program and practical information 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 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 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.

Partners

Irmar LMJL ENS Rennes LMBA LAREMA

Affiliation

ANR CNRS Rennes 1 Rennes 2 Nantes INSA Rennes INRIA ENSRennes UBO UBS Angers UBL