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BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20250129T110000
DTEND;TZID=Europe/Paris:20250129T120000
DTSTAMP:20260417T134641
CREATED:20250124T143655Z
LAST-MODIFIED:20250124T143656Z
UID:18907-1738148400-1738152000@www.math.ens.psl.eu
SUMMARY:Simon André\, raconte-moi le problème de Tarski !
DESCRIPTION:Deux groupes sont dits élémentairement équivalents s’ils vérifient les mêmes énoncés du premier ordre\, c’est-à-dire les mêmes énoncés mathématiques dont les variables désignent uniquement des éléments d’un groupe. Dans les années 40\, Tarski a posé la question suivante : les groupes libres de rang au moins deux sont-ils élémentairement équivalents ? Cette question est longtemps restée ouverte\, et ce n’est qu’au début des années 2000 qu’une réponse affirmative a finalement été apportée par Sela et par Kharlampovich et Myasnikov dans deux séries de travaux volumineuses. Dans mon exposé\, je présenterai ce résultat ainsi que sa généralisation naturelle aux groupes hyperboliques au sens de Gromov.
URL:https://www.math.ens.psl.eu/evenement/simon-andre-raconte-moi-le-probleme-de-tarski/
LOCATION:DMA Salle W
CATEGORIES:Algèbre et géométrie,Séminaire Raconte-moi
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20241211T110000
DTEND;TZID=Europe/Paris:20241211T120000
DTSTAMP:20260417T134641
CREATED:20241122T133854Z
LAST-MODIFIED:20241122T133854Z
UID:18672-1733914800-1733918400@www.math.ens.psl.eu
SUMMARY:Quentin Gazda\, raconte-moi les t-motifs d'Anderson !
DESCRIPTION:En juin 1986\, inspiré par les travaux de Drinfeld\, G. Anderson publie un article fondamental intitulé « t-Motives »\, où il introduit les objets qui portent aujourd’hui son nom. Ce que l’on peut deviner au titre\, c’est qu’Anderson y présente la contrepartie des motifs de Grothendieck en arithmétique des corps de fonctions\, où Fq[t] joue le rôle de Z. Pour autant\, nulle justification n’est donnée quant au choix du nom\, et je me considérerais comme un mathématicien accompli le jour où j’aurai pleinement compris cette analogie. Dans cet exposé\, j’expliquerai ce que j’ai digéré dans cette direction. On présentera les structures classiques de l’arithmétique des corps de fonctions (comme les modules de Drinfeld) tout en insistant sur les nombreuses analogies\, avec la théorie de Hodge p-adique notamment.
URL:https://www.math.ens.psl.eu/evenement/quentin-gazda-raconte-moi-les-t-motifs-danderson/
LOCATION:DMA Salle W
CATEGORIES:Algèbre et géométrie,Séminaire Raconte-moi
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20241127T110000
DTEND;TZID=Europe/Paris:20241127T120000
DTSTAMP:20260417T134641
CREATED:20241122T133440Z
LAST-MODIFIED:20241122T133440Z
UID:18670-1732705200-1732708800@www.math.ens.psl.eu
SUMMARY:Russel Avdek\, tell me about the complex origins of mapping class relations !
DESCRIPTION:I’ll talk about relations between products of Dehn twists along simple closed curves on an oriented surface F. We view these products as elements of the boundary-relative mapping class group of F. A famous example is the `lantern relation’\, discovered by D. Johnson in the 70s by drawing pictures. I’ll describe how many such relations\, such as the lantern\, can be discovered by viewing F as a complex 1-manifold sitting inside of a complex 2-manifold as part of a `Lefschetz fibration’. Time permitting\, I’ll mention higher-dimensional generalizations and open problems.
URL:https://www.math.ens.psl.eu/evenement/russel-avdek-tell-me-about-the-complex-origins-of-mapping-class-relations/
LOCATION:DMA Salle W
CATEGORIES:Algèbre et géométrie,Séminaire Raconte-moi
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20241113T110000
DTEND;TZID=Europe/Paris:20241113T120000
DTSTAMP:20260417T134641
CREATED:20241105T161955Z
LAST-MODIFIED:20241105T162416Z
UID:18585-1731495600-1731499200@www.math.ens.psl.eu
SUMMARY:Vlerë Mehmeti\, raconte-moi l'uniformisation de Koebe--Mumford !
DESCRIPTION:Koebe a démontré un résultat d’uniformisation pour les surfaces de Riemann compactes à travers les « groupes de Schottky ». Ce résultat a été étendu au cadre non archimédien par Mumford. J’expliquerai comment\, en utilisant les « espaces de Berkovich »\, on peut mener une étude uniforme de tous ces objets et de leurs invariants associés.
URL:https://www.math.ens.psl.eu/evenement/vlere-mehmeti-raconte-moi-luniformisation-de-koebe-mumford/
LOCATION:DMA Salle W
CATEGORIES:Algèbre et géométrie,Séminaire Raconte-moi
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20241016T110000
DTEND;TZID=Europe/Paris:20241016T120000
DTSTAMP:20260417T134641
CREATED:20241105T161806Z
LAST-MODIFIED:20241105T162100Z
UID:18582-1729076400-1729080000@www.math.ens.psl.eu
SUMMARY:François Charles\, raconte-moi les surfaces formelles analytiques et les groupes fondamentaux !
DESCRIPTION:Les surfaces arithmétiques formelles-analytiques sont un analogue — mélant arithmétique et analyse complexe — des voisinages tubulaires des courbes dans les surfaces. Je donnerai plusieurs applications de ces objets à des questions de finitude des groupes fondamentaux.
URL:https://www.math.ens.psl.eu/evenement/francois-charles-raconte-moi-les-surfaces-formelles-analytiques-et-les-groupes-fondamentaux-2/
LOCATION:DMA Salle W
CATEGORIES:Algèbre et géométrie,Séminaire Raconte-moi
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20240605T110000
DTEND;TZID=Europe/Paris:20240605T120000
DTSTAMP:20260417T134641
CREATED:20241105T161533Z
LAST-MODIFIED:20241105T161534Z
UID:18578-1717585200-1717588800@www.math.ens.psl.eu
SUMMARY:Bram Petri\, raconte-moi la distribution de Poisson--Dirichlet !
DESCRIPTION:
URL:https://www.math.ens.psl.eu/evenement/bram-petri-raconte-moi-la-distribution-de-poisson-dirichlet/
LOCATION:DMA Salle W
CATEGORIES:Algèbre et géométrie,Séminaire Raconte-moi
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20240529T140000
DTEND;TZID=Europe/Paris:20240529T170000
DTSTAMP:20260417T134641
CREATED:20240513T160313Z
LAST-MODIFIED:20240513T160739Z
UID:17450-1716991200-1717002000@www.math.ens.psl.eu
SUMMARY:Group theory seminar Tiozzo/Freslon/Bordenave
DESCRIPTION:An afternoon on random walks on groups. \n14.00 – 14.45   Giulio Tiozzo (Toronto) « Roots of Alexander polynomials of random positive braids »;   \n15.00 – 15.45   Amaury Freslon (Orsay)  « How to (badly) shuffle cards? »; \n16.15 – 17.00   Charles Bordenave (Marseille) « Strong convergence of matrix algebras and applications to random walks ». \n  \nGiulio Tiozzo  « Roots of Alexander polynomials of random positive braids » \nAs originally observed experimentally by Dehornoy\, roots of Alexander polynomials of random knots display interesting patterns. In this work\, joint with N. Dunfield\, we prove several results on the distribution of such roots in the complex plane\, and discuss further conjectures that originate from them.  \nUsing the Burau representation\, this corresponds to studying random walks on the group SL(2\, C[t]) of 2-by-2 matrices with polynomial coefficients. We compute a sharp lower bound on the probability that such roots lie on the unit circle\, and prove a related central limit theorem. We also show there is a large root-free region near the origin.We introduce the notion of a Lyapunov exponent for the Burau representation\, in the spirit of Deroin-Dujardin\, and a corresponding bifurcation measure\, which we prove to be the limiting measure for the distribution of roots on a region of parameter space. \n  \nAmaury Freslon « How to (badly) shuffle cards? » \nCard shuffling can be modelled by random walks on permutation groups\, and the first example which was studied in depth is the one given by random transpositions. In that case\, Diaconis and Shahshahani proved that the corresponding Markov chain exhibits a so-called cut-off phenomenon. Moreover\, Teyssier recently computed the corresponding cut-off profile\, which is remarkably simple. I will explain how one can similarly define a random walk on the « quantum permutation groups »\, a Hopf algebra which somehow contains the usual permutation groups. I will then report on a joint work with Teyssier and Wang where we prove the cut-off phenomenon for that process and compute the cut-off profile. \n  \nCharles Bordenave « Strong convergence of matrix algebras and applications to random walks » \nWe  will present results on the convergence of the operator norm of random matrices of large dimension. Our random matrices are build by taking tensor products of deterministic matrices and independent Haar distributed unitary matrices or independent random permutation matrices. This class of random matrices allows for example to consider random Schreier graphs of the modular group or of Cartesian products of free groups. We will explain how these convergence results can be used to prove sharp mixing time estimates on random walks.  The talk will be notably based on joint works with Benoit Collins and Hubert Lacoin. \nOrganized by Andrei Alpeev\, Laurent Bartholdi\, Anna Erschler and Panagiotis Tselekidis. \nPartially supported by ERC Advanced Grant 101097307 (P.I.:Laurent Bartholdi).
URL:https://www.math.ens.psl.eu/evenement/group-theory-seminar-tiozzo-freslon-bordenave/
LOCATION:14:00-17:00 Salle W
CATEGORIES:Séminaire de théorie des groupes à l’ENS
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20240416T140000
DTEND;TZID=Europe/Paris:20240416T170000
DTSTAMP:20260417T134641
CREATED:20240404T093514Z
LAST-MODIFIED:20240404T094628Z
UID:17358-1713276000-1713286800@www.math.ens.psl.eu
SUMMARY:Group theory seminar Jaikin-Zapirain/Marquis/Kharlampovich
DESCRIPTION:April 16 (Tuesday) \nFor this seminar we will have the pleasure of listening to three talks:\n\n14.00 – 14.45 Andrei Jaikin-Zapirain (UAM)\, « Compressed subgroups in free groups are inert »\n\n15.00 – 15.45 Timothée Marquis (UCL)\, « Amalgams of rational unipotent groups and residual nilpotence ».\n\n16.15 – 17.00 Olga Kharlampovich (CUNY)\, « Quantification of separability of cubically convex-cocompact subgroups of RAAGs via representations ».\n\n\n\n Andrei Jaikin-Zapirain  « Compressed subgroups in free groups are inert ».  \n Let F be a free group. A finitely generated subgroup H is called compressed in F if it is not contained in a subgroup of F of smaller rank than H\, and it is called inert in F if H ∩ U is compressed in U for any subgroup U of F. In my talk\, I will show that compressed subgroups are also inert. The solves a conjecture of Dicks and Ventura from 1996. \nTimothée Marquis \, « Amalgams of rational unipotent groups and residual nilpotence ». \n Given a group property (P)\, a group G is called residually (P) if every nontrivial element of G has a nontrivial image in some quotient of G that satisfies (P). The study of residual properties of graphs of groups has a long and rich history\, originating from Magnus’ theorem that free groups are residually torsionfree nilpotent. In this talk\, I will start by reviewing a few key results of this history\, before presenting an intriguing phenomenon concerning the residual nilpotence of certain amalgams of rational unipotent groups. Joint work with Pierre-Emmanuel Caprace.  \nOlga Kharlampovich\,   « Quantification of separability of cubically convex-cocompact subgroups of RAAGs via representations« . \n We answer the question asked by Louder\, McReinolds and Patel and prove the following statement. Let L be a RAAG\, H a cubically convex-cocompact subgroup of L\, then there is a finite dimensional representation of L that separates the subgroup H in the induced Zariski topology. As a corollary\, we establish a polynomial upper bound on the size of the quotients used to separate H in L. This implies the same statement for a virtually special group L and\, in particular\, a fundamental group of a hyperbolic 3-manifold. For any finitely generated subgroup H of a limit group L we prove the same results and\, in addition\, show that there exists a finite-index subgroup K containing H\, such that K is a subgroup of a group obtained from H by a series of extensions of centralizers and free products with infinite cyclic group. If H is non-abelian\, the K is fully residually H. A corollary is that a hyperbolic limit group satisfies the Geometric Hanna Neumann conjecture. These are joint results with K. Brown and A. Vdovina. \n\nOrganized by Andrei Alpeev\, Laurent Bartholdi\, Anna Erschler and Panagiotis Tselekidis  \nPartially supported by ERC Advanced Grant 101097307 (P.I.:Laurent Bartholdi).
URL:https://www.math.ens.psl.eu/evenement/group-theory-seminar-jaikin-zapirain-marquis-kharlampovich/
LOCATION:14:00-17:00 Salle W
CATEGORIES:Séminaire de théorie des groupes à l’ENS
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20240403T110000
DTEND;TZID=Europe/Paris:20240403T120000
DTSTAMP:20260417T134641
CREATED:20240119T110825Z
LAST-MODIFIED:20240119T110825Z
UID:17186-1712142000-1712145600@www.math.ens.psl.eu
SUMMARY:Catherine Goldstein\, raconte-moi Charles Hermite !
DESCRIPTION:
URL:https://www.math.ens.psl.eu/evenement/catherine-goldstein-raconte-moi-charles-hermite/
LOCATION:DMA Salle W
CATEGORIES:Algèbre et géométrie,Séminaire Raconte-moi
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20240313T110000
DTEND;TZID=Europe/Paris:20240313T120000
DTSTAMP:20260417T134641
CREATED:20240117T140039Z
LAST-MODIFIED:20240227T161337Z
UID:17183-1710327600-1710331200@www.math.ens.psl.eu
SUMMARY:Cyril Houdayer\, raconte-moi la dynamique des fonctions de type positif des réseaux de rang supérieur !
DESCRIPTION:Je présenterai des résultats d’existence et d’unicité de traces dans l’espace des fonctions de type positif des réseaux de rang supérieur. Je mentionnerai quelques applications à la théorie des représentations unitaires et à la structure de leurs C﹡-algèbres. Ces résultats fournissent des généralisations noncommutatives de théorèmes dus à Margulis\, Stuck—Zimmer et Nevo—Zimmer.
URL:https://www.math.ens.psl.eu/evenement/cyril-houdayer-que-vas-tu-nous-raconter/
LOCATION:DMA Salle W
CATEGORIES:Algèbre et géométrie,Séminaire Raconte-moi
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20240312T140000
DTEND;TZID=Europe/Paris:20240312T170000
DTSTAMP:20260417T134641
CREATED:20240216T150012Z
LAST-MODIFIED:20240216T150238Z
UID:17244-1710252000-1710262800@www.math.ens.psl.eu
SUMMARY:Group theory seminar Houdayer/Chawla/Brofferio
DESCRIPTION:March 12 (Tuesday) \nThis seminar meeting will focus on interactions between groups and random walks\, and will be dedicated to our beloved mentor and colleague\, Anatoly Moiseevich Vershik\, who pioneered this field. \n  \n14.00 – 14.45 Cyril Houdayer (ENS Paris)  « The noncommutative factor theorem for lattices in product groups ». \n15.00 – 15.45   Kunal Chawla (Princeton)   « The Poisson boundary of hyperbolic groups without moment conditions ». \n16.15 – 17.00   Sara Brofferio (Université Paris-Est Créteil Val-de-Marne)\,  « Uniqueness of  invariant measures for random homeomorphisms of the real line ». \n—- \n  \nCyril Houdayer  « The noncommutative factor theorem for lattices in product groups ». \n In this talk\, I will present a noncommutative analogue of Bader-Shalom factor theorem for lattices with dense projections in product groups. Combining with previous works\, this result provides a noncommutative analogue of Margulis factor theorem for all irreducible lattices in higher rank semisimple algebraic groups. Namely\, we give a complete description of all intermediate von Neumann subalgebras sitting between the group von Neumann algebra associated with the lattice and the group measure space von Neumann algebra associated with the action of the lattice on the Furstenberg-Poisson boundary. This is joint work with Rémi Boutonnet. \n  \nKunal Chawla\,  « The Poisson boundary of hyperbolic groups without moment conditions ». \n  \nSara Brofferio   « Uniqueness of  invariant measures for random homeomorphisms of the real line ».  \n A stochastic dynamical systems is a Markov process  defined recursively by X_n=\Psi_n(X_{n-1})=\Psi_n\cdots \Psi_1(X_0) where \Psi_n i.i.d. random continuous transformations on a given space M.  X_n can be seen as the process obtained by the action of the  random walk \Psi_n\cdots \Psi_1 on a (semi)-group Gamma acting on some nice metric space M. \nIn this talk we will focus on stochastic dynamical systems induced  by a random walk on the group of homeomorphisms of R. I will present the results of joint work  with D. Buraczewski and T. Szarek\, in which we establish conditions (relatively optimal) that guarantee that the system admits a unique invariant measure (possibly of infinite mass).
URL:https://www.math.ens.psl.eu/evenement/group-theory-seminar-houdayer-chawla-brofferio/
LOCATION:14:00-17:00 Salle W
CATEGORIES:Séminaire de théorie des groupes à l’ENS
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20240228T110000
DTEND;TZID=Europe/Paris:20240228T120000
DTSTAMP:20260417T134641
CREATED:20240227T161511Z
LAST-MODIFIED:20240227T161646Z
UID:17274-1709118000-1709121600@www.math.ens.psl.eu
SUMMARY:Nalini Anantharaman\, raconte-moi le bas du spectre du laplacien sur les surfaces hyperboliques aléatoires !
DESCRIPTION:Je parlerai de mes travaux avec Laura Monk\, dans lesquels on s’intéresse à la plus petite valeur propre du laplacien sur une surface hyperbolique compacte\, choisie aléatoirement selon la mesure de Weil—Petersson.
URL:https://www.math.ens.psl.eu/evenement/nalini-anantharaman-raconte-moi-le-bas-du-spectre-du-laplacien-sur-les-surfaces-hyperboliques-aleatoires/
LOCATION:DMA Salle W
CATEGORIES:Algèbre et géométrie,Séminaire Raconte-moi
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20240131T140000
DTEND;TZID=Europe/Paris:20240131T170000
DTSTAMP:20260417T134641
CREATED:20240119T142311Z
LAST-MODIFIED:20240123T154931Z
UID:17188-1706709600-1706720400@www.math.ens.psl.eu
SUMMARY:Group theory seminar Berthé/Dang/Duchesne
DESCRIPTION:January 31 (wednesday) \n  \n14.00 – 14.45 Valérie Berthé (Paris VII)\,  « Dendric subshifts  and groups » \n15.00 – 15.45  Nguyen-Bac Dang\, (Orsay) \, « Variation of the Hausdorff dimension of limits set and degenerating Schottky groups » \n16.15 – 17.00  Bruno Duchesne (Orsay)\, TBA \n  \nValérie Berthé\,   « Dendric subshifts  and groups » \n  \nWe discuss a family of  symbolic dynamical systems  that have remarkable  group properties\, the family of dendric words. This family  includes numerous classical families of   symbolic dynamical systems\,   among others codings of interval exchanges. Their return words  form positive   basis of the free group. We   discuss    their dimension groups\, which  are complete invariants of strong orbit equivalence\, andapplications to skew products based on finite  groups. \n\nNguyen Bac Dang\,  « Variation of the Hausdorff dimension of limits set and degenerating Schottky groups » \nIn this talk\, based on a joint work with Vlerë Mehmeti\, I will  explain how one can use some techniques in non-Archimedean geometry to study families of  degenerating complex Schottky groups. More precisely\, each Schottky group comes with a fractal set\, obtained as a limit of an orbit\, called the limit set. We show that under specific conditions\,  one can can obtain an asymptotic formula for  the Hausdorff dimension of the limit set. If time permits\, I will present how certain functions\, called Poincare series have very special  behavior when one works over non-Archimedean fields. \n\n\nOrganized by Andrei Alpeev\, Laurent Bartholdi\, Anna Erschler and Panagiotis Tselekidis  \nPartially supported by ERC Advanced Grant 101097307 (P.I.:Laurent Bartholdi). \n 
URL:https://www.math.ens.psl.eu/evenement/group-theory-seminar-berthe-dang-duchesne/
LOCATION:14:00-17:00 Salle W
CATEGORIES:Séminaire de théorie des groupes à l’ENS
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20240131T110000
DTEND;TZID=Europe/Paris:20240131T120000
DTSTAMP:20260417T134641
CREATED:20240117T135925Z
LAST-MODIFIED:20240119T110650Z
UID:17180-1706698800-1706702400@www.math.ens.psl.eu
SUMMARY:Coline Emprin\, que vas-tu nous raconter ?
DESCRIPTION:
URL:https://www.math.ens.psl.eu/evenement/coline-emprin-que-vas-tu-nous-raconter/
LOCATION:DMA Salle W
CATEGORIES:Algèbre et géométrie,Séminaire Raconte-moi
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20240110T110000
DTEND;TZID=Europe/Paris:20240110T120000
DTSTAMP:20260417T134641
CREATED:20240117T135755Z
LAST-MODIFIED:20240117T135755Z
UID:17177-1704884400-1704888000@www.math.ens.psl.eu
SUMMARY:Stefan Ludwig\, raconte-moi les corps pseudofinis !
DESCRIPTION:
URL:https://www.math.ens.psl.eu/evenement/stefan-ludwig-raconte-moi-les-corps-pseudofinis/
LOCATION:DMA Salle W
CATEGORIES:Algèbre et géométrie,Séminaire Raconte-moi
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20231213T140000
DTEND;TZID=Europe/Paris:20231213T170000
DTSTAMP:20260417T134641
CREATED:20231128T133911Z
LAST-MODIFIED:20231128T134736Z
UID:17033-1702476000-1702486800@www.math.ens.psl.eu
SUMMARY:Group theory seminar Yassawi/Tsankov/Kellendonk
DESCRIPTION:December  13 (wednesday\, unusual day) \n14.00–14.45 Reem Yassawi (Queen Mary University of London) « Tame or wild Toeplitz shifts » \n15.00–15.45 Todor Tsankov (Lyon 1) « Gleason complete flows of locally compact groups » \n\n16.15–17.00 Johannes Kellendonk (Lyon 1) « Which algebraic components of the Ellis semigroup of a non-tame dynamical system are especially big? »\n\n\n\nReem Yassawi     « Tame or wild Toeplitz shifts«  \nThe Ellis semigroup E(X\, T) of a topological dynamical system is defined to be the compactification of the action T in the topology of pointwise convergence on the space of all functions X^X. Tameness is a concept whose roots date back to Rosenthal’s ℓ^1 embedding theorem\, which says that if a sequence in ℓ^1 does not have a weakly Cauchy subsequence\, then it must be the sequence of unit vectors in ℓ^1. Köhler linked the concept of tameness to the Ellis semigroup. A system is tame if its Ellis semigroup has size at most the continuum. Non-tame systems are very far from tame\, as they must contain a copy of βℕ\, the Stone-Čech compactification of ℕ. \nSince then\, the dynamics community has investigated the question of which systems are tame. In this talk I will give a brief exposition of these results\, and talk about work where we study tameness\, or otherwise\, of Toeplitz shifts\, emphasizing the connection between this work and automata. This is joint work with Gabriel Fuhrmann and Johannes Kellendonk. \n\nTodor Tsankov « Gleason complete flows of locally compact groups«  \nThe notion of an irreducible extension of a flow generalizes the one of an almost one-to-one extension (injective on a dense G_delta set)and coincides with the one of a highly proximal extension for minimal flows. The existence of maximal such extensions was proved byAuslander and Glasner in the 70s for minimal flows using an abstract argument\, and a concrete construction using near-ultrafilters wasrecently given by Zucker for arbitrary flows. When the acting group is discrete\, the universal irreducible extension is nothing but the Stonespace of the Boolean algebra of the regular open sets of the space\, already considered by Gleason. We give yet another construction of theuniversal irreducible extension for arbitrary topological groups and prove that for such extensions (which we call Gleason complete) ofa flow of a locally compact group G\, the stabilizer map x -> G_x is continuous (for general flows\, this map is only semi-continuous). Thisis a common generalization of a theorem of Frolík that the set of fixed points of a homeomorphism of a compact\, extremally disconnectedspace is open and a theorem of Veech that the action of a locally compact group on its greatest ambit is free. The theorem implies\, in particular\, that if the action of a locally compact group on its Furstenberg boundary is essentially free\, then it is free. This is joint work with Adrien Le Boudec. \nJohannes Kellendonk « Which algebraic components of the Ellis semigroup of a non-tame dynamical system are especially big?«  \nThe Ellis semigroup E of a group acting by homeomorphisms on a compact space is its compactification in the topology of point wise convergence. It has a lot of interesting structures:  its topology\, the topological properties of its elements\, and its algebraic structure. One property which has incited of lot of interest in recent years is tameness.  In can be characterised in various different ways\, but for our talk the quickest way is to say is that E is tame if its cardinality is at most that of the continuum. So non-tame Ellis semigroups are especially big. We are interested in how this relates to the algebraic structure of the Ellis semigroup. For instance\, when is the kernel of the Ellis semigroup especially big? A recent result shows that\, if the set of idempotents of a minimal right ideal of the Ellis-semigroup of a minimal system is especially big\, then the system cannot be a PI-flow. It is also known that for minimal actions of groups which do not carry an invariant measure\, tameness implies that the system is almost automorphic. In both cases the converse is not true (for almost automorphic non-tame systems see in particular the talk by Reem Yassawi). We will show here that for minimal abelian group actions which are not almost automorphic and whose set of singular points satisfies a condition which will be specified\, the kernel of the Ellis semigroup is especially big\, and here it is in particular the Rees structure group which is especially big. \nOrganized by Andrei Alpeev\, Laurent Bartholdi\, Anna Erschler and Panagiotis Tselekidis  \nPartially supported by ERC Advanced Grant 101097307 (P.I.:Laurent Bartholdi).
URL:https://www.math.ens.psl.eu/evenement/group-theory-seminar-yassawi-tsankov-kellendonk/
LOCATION:14:00-17:00 Salle W
CATEGORIES:Séminaire de théorie des groupes à l’ENS
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20231206T110000
DTEND;TZID=Europe/Paris:20231206T120000
DTSTAMP:20260417T134641
CREATED:20231120T122447Z
LAST-MODIFIED:20240117T135358Z
UID:17008-1701860400-1701864000@www.math.ens.psl.eu
SUMMARY:Rachel Ollivier\, raconte-moi la correspondance de Langlands locale modulo p !
DESCRIPTION:
URL:https://www.math.ens.psl.eu/evenement/rachel-ollivier-que-vas-tu-nous-raconter/
LOCATION:DMA Salle W
CATEGORIES:Algèbre et géométrie,Séminaire Raconte-moi
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20231122T110000
DTEND;TZID=Europe/Paris:20231122T120000
DTSTAMP:20260417T134641
CREATED:20231120T122327Z
LAST-MODIFIED:20231120T122327Z
UID:17001-1700650800-1700654400@www.math.ens.psl.eu
SUMMARY:Eduardo Silva\, raconte-moi les fonctions harmoniques et le bord de Poisson des groupes hyperboliques !
DESCRIPTION:La géométrie asymptotique d’un groupe discret peut être étudiée à partir des espaces de fonctions harmoniques dans le groupe. C’est le cas du bord de Martin\, qui correspond aux fonctions harmoniques positives\, et du bord de Poisson\, qui correspond aux fonctions harmoniques bornées. Dans cet exposé\, nous introduirons ces concepts et expliquerons leurs liens avec les marches aléatoires dans les groupes. Nous discuterons en détail le cas des groupes hyperboliques\, notamment des groupes libres\, et présenterons des résultats qui décrivent le bord de Poisson au travers du bord de Gromov\, avec des hypothèses sur la mesure choisie.
URL:https://www.math.ens.psl.eu/evenement/eduardo-silva-raconte-moi-les-fonctions-harmoniques-et-le-bord-de-poisson-des-groupes-hyperboliques/
LOCATION:DMA Salle W
CATEGORIES:Algèbre et géométrie,Séminaire Raconte-moi
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20231115T140000
DTEND;TZID=Europe/Paris:20231115T164500
DTSTAMP:20260417T134641
CREATED:20231103T125741Z
LAST-MODIFIED:20231103T165612Z
UID:16934-1700056800-1700066700@www.math.ens.psl.eu
SUMMARY:Group theory seminar André/Dong/Rauzy
DESCRIPTION:Nomember 15 (wednesday\, unusual day) \n  \n14.00-14.45 Simon André (Paris VI)  « Sharply 2-transitive infinite finitely generated simple groups ». \n15.00-15.45  Ruiwen Dong (Saarland University)  « Decision problems in sub-semigroups of metabelian groups ». \n16.00-16.45  Emmanuel Rauzy (Munich University)  « Groups with presentations in EDT0L ». \n  \nSimon André  « Sharply 2-transitive infinite finitely generated simple groups ». \nA group G is said to be sharply 2-transitive if it has an action on a set X with at least 2 elements such that\, for all pairs (x\, x’) and (y\, y’) of distinct elements in X\, there exists a unique element g in G such that g(x\, x’) = (y\, y’). For example\, the affine group AGL(1\, K) over a field K is sharply 2-transitive (for its natural action on K)\, and quite surprisingly\, the following question remained open for a long time: does there exist a sharply 2-transitive group that is not isomorphic to some AGL(1\, K)? A few years ago\, Rips\, Segev\, and Tent constructed the first example of a sharply 2-transitive group that is not affine. In my talk\, I will explain that we can go further and construct various sharply 2-transitive groups that are radically different from affine groups. These results were obtained in collaboration with Marco Amelio\, Vincent Guirardel\, and Katrin Tent. \n  \nRuiwen Dong « Decision problems in sub-semigroups of metabelian groups ». \n  \n Algorithmic problems in metabelian groups have been studied as early as the 1950s since the work of Hall. In the 1970s Romanovskii proved decidability of the Group Membership problem (given the generators of a subgroup and a target element\, decide whether the target element is in the subgroup) in metabelian groups. However\, Semigroup Membership (same as Group Membership\, but with sub-semigroups) has been shown to be undecidable in several instances of metabelian groups using embeddings of either the Hilbert’s tenth problem or two-counter automata. \nIn this talk we consider two « intermediate » decision problems: the Identity Problem (deciding if a sub-semigroup contains the neutral element) and the Group Problem (deciding if a sub-semigroup is a group). We reduce them to solving linear equations over the polynomial semiring N[X] and show decidability using an extension of a local-global principle by Einsiedler (2003). \n  \nEmmanuel Rauzy « Groups with presentations in EDT0L » \n  \nThere are numerous connections between group theory and language theory\, which for the most part stem from the fact that elements of a finitely generated group are commonly represented by words on the generators. \nOut of these connections\, one of the least studied ones is the notion of a group that admits a presentation in a given class of languages. Indeed\, while the notions of finite presentations and of recursive presentations are commonly invoked\, finite languages and recursive languages correspond to the two extremes of the Chomsky hierarchy -there is much in between! \nWe show that the groups that admit an L-presentation\, a notion introduced by Bartholdi in 2000\, correspond exactly to those that admit EDT0L presentations\, a class of language which has been the focus of much attention in group theory following work of Ciobanu and Elder. \nWe present a uniform proof for the fact that one can compute finite\, nilpotent\, metabelian and free quotients of a group described by an EDT0L presentation\, extending results of Bartholdi\, Eick\, Hartung. This proof relies on subgroup functors that satisfy some Noetherianity conditions. \nFinally\, we explain how these results allow us to produce examples of recursively presented groups that do not admit EDT0L presentations. \nThis is joint work with Laurent Bartholdi and Leon Pernak.
URL:https://www.math.ens.psl.eu/evenement/group-theory-seminar-andre-dong-rauzy/
LOCATION:14:00-17:00 Salle W
CATEGORIES:Séminaire de théorie des groupes à l’ENS
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20231018T110000
DTEND;TZID=Europe/Paris:20231018T120000
DTSTAMP:20260417T134641
CREATED:20231016T091956Z
LAST-MODIFIED:20231016T092543Z
UID:16900-1697626800-1697630400@www.math.ens.psl.eu
SUMMARY:Muriel Livernet\, raconte-moi les multicomplexes !
DESCRIPTION:Les multicomplexes sont des outils d’algèbre homologique qui généralisent la notion de bicomplexes. Déjà présents sous diverses formes dans les travaux de Wall (pour des résolutions d’extensions de groupes) ou de Liulevicius en algèbre homologique\, ils sont régulièrement présents dans la littérature comme outil efficace pour calculer des groupes d’homologie. Plus récemment on les rencontre dans le calcul d’invariants homologiques de variétés. L’objectif de mon exposé est d’introduire les multicomplexes\, les suites spectrales associées ainsi qu’une théorie d’homotopie pour les multicomplexes.
URL:https://www.math.ens.psl.eu/evenement/muriel-livernet-raconte-moi-les-multicomplexes/
LOCATION:DMA Salle W
CATEGORIES:Algèbre et géométrie,Séminaire Raconte-moi
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20231010T140000
DTEND;TZID=Europe/Paris:20231010T170000
DTSTAMP:20260417T134641
CREATED:20231003T082741Z
LAST-MODIFIED:20231107T143704Z
UID:16827-1696946400-1696957200@www.math.ens.psl.eu
SUMMARY:Timothée Bénard / Panagiotis Tselekidis / Andrey Alpeev
DESCRIPTION:14.00 — 14.45 Timothée Bénard (University of Warwick)\, « Limit theorems on nilpotent Lie groups ».\n15.00 — 15.45 Panagiotis Tselekidis (ENS Paris)\,  « Asymptotic dimension of finitely generated groups ».\n16.15 — 17.00 Andrey Alpeev (ENS Paris)\, « Invariant random order extension and amenability ».
URL:https://www.math.ens.psl.eu/evenement/timothee-benard-panagiotis-tselekidis-andrey-alpeev/
LOCATION:DMA Salle W
CATEGORIES:Séminaire de théorie des groupes à l’ENS
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20230516T140000
DTEND;TZID=Europe/Paris:20230516T170000
DTSTAMP:20260417T134641
CREATED:20230503T161405Z
LAST-MODIFIED:20230503T161405Z
UID:16435-1684245600-1684256400@www.math.ens.psl.eu
SUMMARY:Un après-midi de théorie des groupes
DESCRIPTION:Le séminaire sera dans salle W et retransmis sur Zoom : \nhttps://us02web.zoom.us/j/82734674940\nMeeting ID: 827 3467 4940\nMot de passe:\nG est un Graphe de Cayley du groupe libre à 107 générateurs. Quel est le degré de ce graphe? Tapez le numéro à trois chiffres comme un mot de passe. \n  \n14.00 -- 14.45 Jingyin Huang (Ohio State University)\, Labeled four wheels\nand the K(pi\,1) problem for reflection arrangement complements\n\n15.00 -- 15.45 Corentin Bodard (University of Geneva)\, Intermediate\ngeodesic growth in virtually nilpotent groups\n\n16.00 -- 16.45 Lopez Neumann Antonio (École polytechnique)\, Vanishing of\nthe second L^p-cohomology group for most semisimple groups of rank at\nleast 3
URL:https://www.math.ens.psl.eu/evenement/un-apres-midi-de-theorie-des-groupes-8/
LOCATION:DMA Salle W
CATEGORIES:Séminaire de théorie des groupes à l’ENS
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20230411T140000
DTEND;TZID=Europe/Paris:20230411T160000
DTSTAMP:20260417T134641
CREATED:20230401T105150Z
LAST-MODIFIED:20230410T154506Z
UID:16349-1681221600-1681228800@www.math.ens.psl.eu
SUMMARY:Un après-midi de théorie des groupes
DESCRIPTION:Le séminaire sera dans salle W et retransmis sur Zoom : \nhttps://us02web.zoom.us/j/83495836865\nMeeting ID: 834 9583 6865\nMot de passe:\nG est un Graphe de Cayley du groupe libre à 107 générateurs. Quel est le degré de ce graphe? Tapez le numéro à trois chiffres comme un mot de passe. \n14.00 -- 14.45 Alex Eskin (University of Chicago)\, On a theorem of\nFurstenberg\n\n15.00 -- 15.45 (CANCELLED!) Alex Gamburd (Graduate Center\, CUNY)\, Arithmetic and\nDynamics on Varieties of Markoff Type \n\n15.00 -- 15.45 Vadim Kaimanovich (University of Ottawa)\, Limit\ndistributions of branching random walks
URL:https://www.math.ens.psl.eu/evenement/un-apres-midi-de-theorie-des-groupes-7/
LOCATION:DMA Salle W
CATEGORIES:Séminaire de théorie des groupes à l’ENS
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20230314T140000
DTEND;TZID=Europe/Paris:20230314T170000
DTSTAMP:20260417T134641
CREATED:20230228T210247Z
LAST-MODIFIED:20230228T210247Z
UID:16316-1678802400-1678813200@www.math.ens.psl.eu
SUMMARY:Un après-midi de théorie des groupes
DESCRIPTION:Le séminaire sera dans salle W et retransmis sur Zoom : \nhttps://us02web.zoom.us/j/83940344109\nMeeting ID: 839 4034 4109\nMot de passe:\nG est un Graphe de Cayley du groupe libre à 107 générateurs. Quel est le degré de ce graphe? Tapez le numéro à trois chiffres comme un mot de passe. \n14.00-14.45 Emmanuel Breuillard (Oxford)\, Random character varieties.\n\n15.00-15.45 Pierre Py (Strasbourg)\, Subgroups of hyperbolic groups\,\nfiniteness properties and complex hyperbolic lattices.\n\n16.00-16.45 Olga Kharlampovich (Graduate Center and Hunter College\nCUNY)\,  Equations and first-order sentences in random groups.
URL:https://www.math.ens.psl.eu/evenement/un-apres-midi-de-theorie-des-groupes-6/
LOCATION:DMA Salle W
CATEGORIES:Séminaire de théorie des groupes à l’ENS
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20230222T140000
DTEND;TZID=Europe/Paris:20230222T170000
DTSTAMP:20260417T134641
CREATED:20230215T213034Z
LAST-MODIFIED:20230215T213034Z
UID:16293-1677074400-1677085200@www.math.ens.psl.eu
SUMMARY:Un après-midi de théorie des groupes
DESCRIPTION:Le séminaire sera dans salle W et retransmis sur Zoom : \nhttps://https://us02web.zoom.us/j/81313173812\nMeeting ID: 813 1317 3812\nMot de passe:\nG est un Graphe de Cayley du groupe libre à 107 générateurs. Quel est le degré de ce graphe? Tapez le numéro à trois chiffres comme un mot de passe. \n14.00-14.45 Yves Benoist (Orsay)\, Harmonic functions on the Heisenberg\ngroup\n\n15.00-15.45 Nora  Szakacs (Manchester)\, Inverse semigroups as metric\nspaces\, and their uniform Roe algebras.\n\n16.00-16.45 Richard Aoun (Marne-la-Vallée)\,  Concentration\ninequalities for random walks on hyperbolic spaces\, and applications.
URL:https://www.math.ens.psl.eu/evenement/un-apres-midi-de-theorie-des-groupes-5/
LOCATION:ENS (salle W)
CATEGORIES:Séminaire de théorie des groupes à l’ENS
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20230110T140000
DTEND;TZID=Europe/Paris:20230110T170000
DTSTAMP:20260417T134641
CREATED:20230106T173826Z
LAST-MODIFIED:20230106T173826Z
UID:16229-1673359200-1673370000@www.math.ens.psl.eu
SUMMARY:Un après-midi de théorie des groupes
DESCRIPTION:Le séminaire sera dans salle W et retransmis sur Zoom : \nhttps://us02web.zoom.us/j/83858227469\nMeeting ID: 838 5822 7469 \nMot de passe:\nG est un Graphe de Cayley du groupe libre à 107 générateurs. Quel est le degré de ce graphe? Tapez le numéro à trois chiffres comme un mot de passe. \n14.00 – 14.45 Greg Kuperberg (UC Davis & IHES)\, An obstruction to quantum algorithms from small cancellation theory \n15.00 – 15.45 Konstantinos Tsouvalas (IHES )\, Linear hyperbolic groups indiscrete in rank 1 and products \n16.00 – 16.45 Delaram Kahrobaei (Cuny & IHES)\, Applied Group Theory in the Quantum and Artificial Intelligence Era
URL:https://www.math.ens.psl.eu/evenement/un-apres-midi-de-theorie-des-groupes-4/
LOCATION:ENS (salle W)
CATEGORIES:Algèbre et géométrie,Séminaire de théorie des groupes à l’ENS
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20221220T140000
DTEND;TZID=Europe/Paris:20221220T170000
DTSTAMP:20260417T134641
CREATED:20221216T214153Z
LAST-MODIFIED:20221216T214153Z
UID:16209-1671544800-1671555600@www.math.ens.psl.eu
SUMMARY:Un après-midi de théorie des groupes
DESCRIPTION:Le séminaire sera dans salle W et retransmis sur Zoom : \nhttps://us02web.zoom.us/j/89219979639\nMeeting ID: 892 1997 9639\nMot de passe:\nG est un Graphe de Cayley du groupe libre à 107 générateurs. Quel est\nle degré de ce graphe? Tapez le numéro à trois chiffres comme un mot\nde passe. \n14.00 -- 14.45 Jim Belk (University of Glasgow)\,\n"Embeddings into Finitely Presented Simple Groups" (in presence\, room W)\n\n15.00 -- 15.45 Mark Pengitore (University of Virginia)\, "Characteristic\nquotients of surface groups and residual finiteness of mapping class\ngroups" (online\, shown on the screen)\n\n16.00 -- 16.45 Nikolay Bogachev (Weizmann Institute\, IITP RAS)\, "On geometry\nand arithmetic of hyperbolic orbifolds" (TBC\, online\, shown on the screen)
URL:https://www.math.ens.psl.eu/evenement/un-apres-midi-de-theorie-des-groupes-3/
LOCATION:14:00-17:00 Salle W
CATEGORIES:Séminaire de théorie des groupes à l’ENS
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20221209T160000
DTEND;TZID=Europe/Paris:20221209T173000
DTSTAMP:20260417T134641
CREATED:20221206T135217Z
LAST-MODIFIED:20221206T135217Z
UID:16179-1670601600-1670607000@www.math.ens.psl.eu
SUMMARY:Integer points on analytic sets
DESCRIPTION:In 2004 I proved that that if C is a transcendental curve definable in the structure R_{an}\, then the number of points on C with integer coordinates of modulus less than H\, is bounded by k loglog H for some constsnt k depending only on C. (The situation is vastly different for rational points.) The proof used the fact that such sets C are\, in fact\, semi-analytic everywhere-including infinity-and so the crux of the matter was to bound the number of solutions to equations of the form \n(*)    F(1/n) = 1/mfor n\, m integers bounded in modulus by (large) H\, and where F is a non-algebraic\, analytic function defined on an open interval containing 0.\nIt turns out that there is probably no generalization of the 2004 result for arbitrary R_{an}-definable sets (which need not be globally\, or even locally\, semi-analytic) but inspired by observations of Gareth Jones and Gal Binyamini\, the three of us began looking at equations of the form (*) in many variables and I shall be reporting on our results.
URL:https://www.math.ens.psl.eu/evenement/integer-points-on-analytic-sets/
LOCATION:Salle W (Toits du DMA)
CATEGORIES:Séminaire Géométrie et théorie des modèles
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20221209T141500
DTEND;TZID=Europe/Paris:20221209T154500
DTSTAMP:20260417T134641
CREATED:20221206T135049Z
LAST-MODIFIED:20221206T135049Z
UID:16177-1670595300-1670600700@www.math.ens.psl.eu
SUMMARY:Un théorème de finitude pour les fonctions tropicales sur les squelettes
DESCRIPTION:Les squelettes sont des sous-ensembles linéaires par morceaux d’espaces analytiques non-archimédiens apparaissant naturellement dans nombre de situations. Nous présenterons un résultat général de finitude\, obtenu en collaboration avec A. Ducros\, E. Hrushovski et J. Ye\, concernant le groupe abélien ordonné des fonctions tropicales sur les squelettes des analytifiés de Berkovich de variétés algébriques. Notre approche utilise la version modèle théorique de l’analytification (la complétion stable) développée dans un travail antérieur avec E. Hrushovski.
URL:https://www.math.ens.psl.eu/evenement/un-theoreme-de-finitude-pour-les-fonctions-tropicales-sur-les-squelettes/
LOCATION:Salle W (Toits du DMA)
CATEGORIES:Séminaire Géométrie et théorie des modèles
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20221125T160000
DTEND;TZID=Europe/Paris:20221125T173000
DTSTAMP:20260417T134641
CREATED:20221122T152916Z
LAST-MODIFIED:20221122T152916Z
UID:16127-1669392000-1669397400@www.math.ens.psl.eu
SUMMARY:On non-Diophantine sets in rings of functions
DESCRIPTION:For a ring R\, a subset of a cartesian power of R is said to be Diophantine if it is positive existentially definable over R with parameters from R. In general\, Diophantine sets over rings are not well-understood even in very natural situations; for instance\, we do not know if the ring of integers Z is Diophantine in the field of rational numbers. To show that a set is Diophantine requires to produce a particular existential formula that defines it. However\, to show that a set is not Diophantine is a more subtle task; in lack of a good description of Diophantine sets it requires to find at least a property shared by all of them. I will give an outline of some recent joint work with Garcia-Fritz and Pheidas on showing that several sets and relations over rings of polynomials and rational functions that are not Diophantine.
URL:https://www.math.ens.psl.eu/evenement/on-non-diophantine-sets-in-rings-of-functions/
LOCATION:IHP salle 01 et zoom
CATEGORIES:Séminaire Géométrie et théorie des modèles
END:VEVENT
END:VCALENDAR