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X-WR-CALDESC:évènements pour Département de mathématiques et applications
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BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20240131T140000
DTEND;TZID=Europe/Paris:20240131T170000
DTSTAMP:20260417T184111
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:20260417T184111
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:20260417T184111
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:20260417T184111
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:20260417T184111
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:20260417T184111
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:20260417T184111
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:20260417T184111
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:20260417T184111
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:20260417T184111
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:20260417T184111
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:20260417T184111
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:20260417T184111
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:20260417T184111
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:20260417T184111
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:20260417T184111
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:20260417T184111
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:20260417T184111
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
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20221125T141500
DTEND;TZID=Europe/Paris:20221125T154500
DTSTAMP:20260417T184111
CREATED:20221122T152504Z
LAST-MODIFIED:20221122T152638Z
UID:16123-1669385700-1669391100@www.math.ens.psl.eu
SUMMARY:Abundance of strongly minimal autonomous differential equations
DESCRIPTION:In several classical families of differential equations such as the Painlevé families (Nagloo\, Pillay) or finite dimensional families of Schwarzian differential equations (Blazquez-Sanz\, Casale\, Freitag\, Nagloo)\, the following picture has been obtained regarding the transcendence properties of their solutions: \n– (Strong minimality): outside of an exceptional set of parameters\, the corresponding differential equations are strongly minimal\,\n– (Geometric triviality): algebraic independence of several solutions is controlled by pairwise algebraic independence outside of this exceptional set of parameters\,\n– (Multidimensionality): the differential equations defined by generic independent parameters are orthogonal. \nAre the families of differential equations satisfying such transcendence properties scarce or abundant in the universe of algebraic differential equations? \nI will describe an abundance result for families of autonomous differential equations satisfying the first two properties. The model-theoretic side of the proof uses a fine understanding of the structure of autonomous differential equations internal to the constants that we have recently obtained in a joint work with Rahim Moosa. The geometric side of the proof uses a series of papers of S.C. Coutinho and J.V. Pereira on the dynamical properties of a generic foliation.
URL:https://www.math.ens.psl.eu/evenement/abundance-of-strongly-minimal-autonomous-differential-equations/
LOCATION:IHP salle 01
CATEGORIES:Séminaire Géométrie et théorie des modèles
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20221125T110000
DTEND;TZID=Europe/Paris:20221125T123000
DTSTAMP:20260417T184111
CREATED:20221122T152309Z
LAST-MODIFIED:20221122T152714Z
UID:16121-1669374000-1669379400@www.math.ens.psl.eu
SUMMARY:Taming perfectoid fields
DESCRIPTION:Tilting perfectoid fields\, developed by Scholze\, allows to transfer results between certain henselian fields of mixed characteristic and their positive characteristic counterparts and vice versa. We present a model-theoretic approach to tilting via ultraproducts\, which allows to transfer many first-order properties between a perfectoid field and its tilt (and conversely). In particular\, our method yields a simple proof of the Fontaine-Wintenberger Theorem which states that the absolute Galois group of a perfectoid field and its tilt are canonically isomorphic. A key ingredient in our approach is an Ax-Kochen/Ershov principle for perfectoid fields (and generalizations thereof).\nThis is joint work with Konstantinos Kartas.
URL:https://www.math.ens.psl.eu/evenement/taming-perfectoid-fields/
LOCATION:IHP salle 01
CATEGORIES:Séminaire Géométrie et théorie des modèles
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20221122T140000
DTEND;TZID=Europe/Paris:20221122T170000
DTSTAMP:20260417T184111
CREATED:20221118T212841Z
LAST-MODIFIED:20221118T213000Z
UID:16113-1669125600-1669136400@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/86001581637\nMeeting ID: 860 0158 1637\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 Anne Lonjou  (UPV/EHU\, University of the Basque Country)\,\n"Cremona goup over finite fields and Neretin groups"\n\n15.00 -- 15.45 Eduardo Silva (ENS\, Paris)\, "Dead ends on wreath products\nand lamplighter groups"\n\n16.00 -- 16.45 Alina Vdovina (CUNY\, New York)\, "Higher structures in\nmathematics: buildings\, k-graphs and C*-algebras"
URL:https://www.math.ens.psl.eu/evenement/16113/
LOCATION:14:00-17:00 Salle W
CATEGORIES:Séminaire de théorie des groupes à l’ENS
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20221019T140000
DTEND;TZID=Europe/Paris:20221019T170000
DTSTAMP:20260417T184111
CREATED:20221019T085810Z
LAST-MODIFIED:20221019T085810Z
UID:16061-1666188000-1666198800@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/83927866358\nMeeting ID: 839 2786 6358 \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 Yury Neretin (University of Vienna)\, « Infinite symmetric\ngroups and cobordisms of triangulated surfaces » \n15.00 – 15.45 Matteo Tarocchi (University of Milano-Biocca)\,\n« Thompson-like groups acting on fractals » \n16.00 – 16.45 Rachel Skipper (ENS)\, « Maximal Subgroups of Thompson’s group V » \n 
URL:https://www.math.ens.psl.eu/evenement/un-apres-midi-de-theorie-des-groupes-2/
LOCATION:14:00-17:00 Salle W
CATEGORIES:Séminaire de théorie des groupes à l’ENS
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20220629T110000
DTEND;TZID=Europe/Paris:20220629T120000
DTSTAMP:20260417T184111
CREATED:20220620T113932Z
LAST-MODIFIED:20220627T193238Z
UID:15682-1656500400-1656504000@www.math.ens.psl.eu
SUMMARY:Anthony Várilly Alvarado\, raconte-moi comment utiliser la géométrie pour construire des codes pour le stockage à grande échelle !
DESCRIPTION:Motivated by large-scale storage problems around data loss\, a budding branch of coding theory has surfaced in the last decade or so\, centered around locally recoverable codes.  A code is a subset of a finite-dimensional vector space over a finite field\, chosen carefully so that all its elements are locally isolated\, as if they were « repelling » each other.  Each vector in a code is called a code word.  Locally recoverable codes have the property that individual entries in a code word are functions of other entries in the same word.  If an entry is accidentally lost\, it can be recomputed\, and hence a code word can be repaired.  Algebraic geometry has a role to play in the design of codes with locality properties. In this talk I will explain how to use algebraic surfaces to both reinterpret constructions of optimal codes already found in the literature\, and to find new locally recoverable codes\, many of which are optimal.
URL:https://www.math.ens.psl.eu/evenement/anthony-vrilly-alvarado-que-vas-tu-nous-raconter/
LOCATION:En salle W au DMA\, ou sur Zoom
CATEGORIES:Algèbre et géométrie,Séminaire Raconte-moi
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20220614T160000
DTEND;TZID=Europe/Paris:20220614T173000
DTSTAMP:20260417T184111
CREATED:20220607T105456Z
LAST-MODIFIED:20220607T105456Z
UID:15638-1655222400-1655227800@www.math.ens.psl.eu
SUMMARY:On the Borel complexity of modules
DESCRIPTION:We prove that among all countable\, commutative rings R (with unit) the theory of R-modules is not Borel complete if and only if there are only countably many non-isomorphic countable R-modules. From the proof\, we obtain a succinct proof that the class of torsion free abelian groups is Borel complete.\nThe results above follow from some general machinery that we expect to have applications in other algebraic settings. Here\, we also show that for an arbitrary countable ring R\, the class of left R-modules equipped with an endomorphism is Borel complete; as is the class of left R-modules equipped with predicates for four submodules. This is joint work with D. Ulrich.
URL:https://www.math.ens.psl.eu/evenement/on-the-borel-complexity-of-modules/
LOCATION:Sophie Germain salle 1016.
CATEGORIES:Théorie des Modèles et Groupes
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20220607T160000
DTEND;TZID=Europe/Paris:20220607T173000
DTSTAMP:20260417T184111
CREATED:20220603T082812Z
LAST-MODIFIED:20220603T082812Z
UID:15632-1654617600-1654623000@www.math.ens.psl.eu
SUMMARY:Cercles isométriques mais contractiles dans les cônes asymptotiques des groupes
DESCRIPTION:La contractilité de tous les cercles dans les cônes asymptotiques d’un groupe G de type fini implique que G est de présentation finie avec fonction de Dehn au plus polynomiale.  Le distorsion métrique de tous ces cercles est une propriété plus forte qui implique que G est fortement raccourci (“strongly shortcut”).  La propriété fortement raccourci est satisfaite par diverses familles de groupes de courbure négative ou nulle\, notamment les groupes hyperboliques\, CAT(0)\, Helly\, et systoliques\, mais elle est aussi satisfaite par le groupe de Heisenberg discret.\n    Je discuterai d’un récent travail en commun avec Cashen et Woodhouse où on a montré qu’une famille infinie de groupes flocon de\nneige (“snowflake groups”) ont des cônes asymptotiques simplement connexes mais des graphes de Cayley qui ne sont pas fortement\nraccourcis.  Ce sont les premiers exemples de groupes qui ont des cônes asymptotiques dans lesquels il existe des cercles isométriques\nmais contractiles.
URL:https://www.math.ens.psl.eu/evenement/cercles-isometriques-mais-contractiles-dans-les-cones-asymptotiques-des-groupes/
LOCATION:salle 1016 Sophie Germain
CATEGORIES:Théorie des Modèles et Groupes
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20220531T160000
DTEND;TZID=Europe/Paris:20220531T170000
DTSTAMP:20260417T184111
CREATED:20220502T091657Z
LAST-MODIFIED:20220502T091657Z
UID:15552-1654012800-1654016400@www.math.ens.psl.eu
SUMMARY:Le théorème du corps gauche de Zilber / Zilber's Skew-Field Theorem (joint with Frank Wagner)
DESCRIPTION:Le théorème du corps est l’observation qu’un groupe de rang de Morley fini connexe\, résoluble\, et non nilpotent\, interprète un corps infini. Par d’autres résultats classiques\, le corps est commutatif et même algébriquement clos.\nLe théorème du corps est souvent vu comme corollaire du «théorème d’engendrement par des indécomposables» mais c’est une erreur car il en est indépendant. Il a quelques variantes\, des théorèmes de linéarisation d’actions de groupes.\nJe donnerai un énoncé qui généralise naturellement tous les résultats «à la Zilber». C’est un résultat de linéarisation de bimodules\, dans un contexte plus général que les théories de rang de Morley fini. En général on interprète un corps gauche.\nPrérequis : notion de définissabilité ; «lemme de Schur» en théorie des représentations (l’anneau des endomorphismes qui commutent avec une représentation irréductible est en fait un corps gauche). \nZilber’s Field Theorem ZFT is the observation that a connected\, soluble\, non-nilpotent group of finite Morley rank interprets an infinite field. By other classical results\, the field is commutative indeed\, and even algebraically closed.\nThe ZFT is often seen as a corollary to Zilber’s `indecomposable generation theorem’; but it actually is independent from it. The ZFT has a couple of variants\, linearisation results for definable group actions.\nI shall give a theorem which generalises naturally all results `à la Zilber’. It is a tool that can linearise bimodule actions\, in a broader context than theories of finite Morley rank. In general it produces a definable skew-field.\nPrerequisites: definable sets; `Schur’s lemma’ from representation theory (the ring of endomorphisms commuting with an irreducible representation\, actually is a skew-field).
URL:https://www.math.ens.psl.eu/evenement/le-theoreme-du-corps-gauche-de-zilber-zilbers-skew-field-theorem-joint-with-frank-wagner/
LOCATION:Sophie Germain salle 1016.
CATEGORIES:Théorie des Modèles et Groupes
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20220513T160000
DTEND;TZID=Europe/Paris:20220513T173000
DTSTAMP:20260417T184111
CREATED:20220503T102922Z
LAST-MODIFIED:20220503T102922Z
UID:15562-1652457600-1652463000@www.math.ens.psl.eu
SUMMARY:Sharp o-minimality: towards an arithmetically tame geometry
DESCRIPTION:Over the last 15 years a remarkable link between o-minimality and algebraic/arithmetic geometry has been unfolding following the discovery of Pila-Wilkie’s counting theorem and its applications around unlikely intersections\, functional transcendence etc. While the counting theorem is nearly optimal in general\, Wilkie has conjectured a much sharper form in the structure R_exp. There is a folklore expectation that such sharper bounds should hold in structures « coming from geometry »\, but for lack of a general formalism explicit conjectures have been made only for specific structures.\nI will describe a refinement of the standard o-minimality theory aimed at capturing the finer « arithmetic tameness » that we expect to see in structures coming from geometry. After presenting the general framework I will discuss my result with Vorobjov showing that the restricted Pfaffian structure is sharply o-minimal\, and how this was used in our recent work with Novikov and Zack to prove Wilkie’s conjecture for the restricted Pfaffian structure and for Wilkie’s original case of R_exp. I will also discuss some conjectures on the construction of larger sharply o-minimal structures\, and some partial results in this direction. Finally I will explain the crucial role played by these results in my recent work with Schmidt and Yafaev on Galois orbit lower bounds for CM points in general Shimura varieties\, and subsequently in the recent resolution of general André-Oort conjecture by Pila-Shankar-Tsimerman-(Esnault-Groechenig).
URL:https://www.math.ens.psl.eu/evenement/sharp-o-minimality-towards-an-arithmetically-tame-geometry/
LOCATION:Salle W (ENS) et Zoom
CATEGORIES:Séminaire Géométrie et théorie des modèles
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20220513T141500
DTEND;TZID=Europe/Paris:20220513T154500
DTSTAMP:20260417T184111
CREATED:20220421T120028Z
LAST-MODIFIED:20220503T103029Z
UID:15542-1652451300-1652456700@www.math.ens.psl.eu
SUMMARY:Skew-invariant curves and algebraic independence
DESCRIPTION:A σ-variety over a difference field (K\,σ) is a pair (X\,φ) consisting of an algebraic variety X over K and φ:X → X^σ is a regular map from X to its transform Xσ under σ. A subvariety Y ⊆ X is skew-invariant if φ(Y) ⊆ Y^σ. In earlier work with Alice Medvedev we gave a procedure to describe skew-invariant varieties of σ-varieties of the form (𝔸^n\,φ) where φ(x_1\,…\,x_n) = (P_1(x_1)\,…\,P_n(x_n)). The most important case\, from which the others may be deduced\, is that of n = 2. In the present work we give a sharper description of the skew-invariant curves in the case where P_2 = P_1^τ for some other automorphism of K which commutes with σ. Specifically\, if P in K[x] is a polynomial of degree greater than one which is not eventually skew-conjugate to a monomial or ± Chebyshev (i.e. P is “nonexceptional”) then skew-invariant curves in (𝔸^2\,(P\,P^τ)) are horizontal\, vertical\, or skew-twists: described by equations of the form y = α^{σ^n} ∘ P^{σ^{n-1}} ∘ ⋅⋅⋅ ∘ P^σ ∘ P(x) or x = β^{σ{-1}}∘ P^{τ σ^{-n-2}}∘ P^{τ σ^{-n-3}}∘ ⋅⋅⋅ ∘ P^τ(y) where P = α ∘ β and P^τ = α^{σ^{n+1}}∘ β^{σ^n}} for some integer n.
URL:https://www.math.ens.psl.eu/evenement/skew-invariant-curves-and-algebraic-independence/
CATEGORIES:Séminaire Géométrie et théorie des modèles
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20220513T110000
DTEND;TZID=Europe/Paris:20220513T123000
DTSTAMP:20260417T184111
CREATED:20220421T115851Z
LAST-MODIFIED:20220503T103102Z
UID:15539-1652439600-1652445000@www.math.ens.psl.eu
SUMMARY:Complexity of l-adic sheaves
DESCRIPTION:To a complex of l-adic sheaves on a quasi-projective variety one associate an integer\, its complexity. The main result on the complexity is that it is continuous with tensor product\, pullback and pushforward\, providing effective version of the constructibility theorems in l-adic cohomology. Another key feature is that the complexity bounds the dimensions of the cohomology groups of the complex. This can be used to prove equidistribution results for exponential sums over finite fields. This is due to Will Sawin\, written up in collaboration with Javier Fresán and Emmanuel Kowalski.
URL:https://www.math.ens.psl.eu/evenement/complexity-of-l-adic-sheaves/
CATEGORIES:Séminaire Géométrie et théorie des modèles
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20220510T160000
DTEND;TZID=Europe/Paris:20220510T173000
DTSTAMP:20260417T184111
CREATED:20220502T091359Z
LAST-MODIFIED:20220502T115516Z
UID:15549-1652198400-1652203800@www.math.ens.psl.eu
SUMMARY:Existential theories of henselian fields\, parameters welcome
DESCRIPTION:The first-order theories of local fields of positive characteristic\, i.e. fields of Laurent series over finite fields\, are far less well understood than their characteristic zero analogues: the fields of real\, complex and p-adic numbers. On the other hand\, the existential theory of an equicharacteristic henselian valued field in the language of valued fields is controlled by the existential theory of its residue field. One is decidable if and only if the other is decidable. When we add a parameter to the language\, things get more complicated. Denef and Schoutens gave an algorithm\, assuming resolution of singularities\, to decide the existential theory of rings like Fp[[t]]\, with the parameter t in the language. I will discuss their algorithm and present a new result (from ongoing work\, with Dittmann and Fehm) that weakens the hypothesis to a form of local uniformization\, and which works in greater generality.
URL:https://www.math.ens.psl.eu/evenement/tba-12/
LOCATION:Sophie Germain salle 1016
CATEGORIES:Théorie des Modèles et Groupes
END:VEVENT
END:VCALENDAR