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X-ORIGINAL-URL:https://www.math.ens.psl.eu
X-WR-CALDESC:évènements pour Département de mathématiques et applications
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TZID:Europe/Paris
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TZNAME:CEST
DTSTART:20210328T010000
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DTSTART:20211031T010000
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DTSTART:20220327T010000
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BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20220318T140000
DTEND;TZID=Europe/Paris:20220318T153000
DTSTAMP:20260418T161447
CREATED:20220311T152633Z
LAST-MODIFIED:20220311T152633Z
UID:15421-1647612000-1647617400@www.math.ens.psl.eu
SUMMARY:Interdefinability and compatibility in certain o-minimal expansions of the real field
DESCRIPTION:Let us say that a real function f is o-minimal if the expansion (R\,f) of the real field by f is o-minimal. A function g is definable from f if g is definable in (R\,f). Two o-minimal functions are compatible if there exists an o-minimal expansion M of the real field in which they are both definable. I will discuss the o-minimality\, the interdefinability and the compatibility of two special functions\, Euler’s Gamma and Riemann’s Zeta\, restricted to the reals. If time allows it\, I will present a general technique for establishing whether a function is definable or not in a given o-minimal expansion of the reals. Joint work with J.-P. Rolin and P. Speissegger.
URL:https://www.math.ens.psl.eu/evenement/interdefinability-and-compatibility-in-certain-o-minimal-expansions-of-the-real-field/
LOCATION:Zoom
CATEGORIES:Séminaire Géométrie et théorie des modèles
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20220315T160000
DTEND;TZID=Europe/Paris:20220315T173000
DTSTAMP:20260418T161447
CREATED:20220309T160534Z
LAST-MODIFIED:20220309T160950Z
UID:15402-1647360000-1647365400@www.math.ens.psl.eu
SUMMARY:Curve-excluding fields
DESCRIPTION:Consider the class of fields with Char(K)=0 and x^4+y^4=1 has only 4 solutions in K\, we show that this class has a model companion\, which we denote by curve-excluding fields. Curve-excluding fields provides (counter)examples to various questions. Model theoretically\, they are model complete and TP_2. Field theoretically\, they are not large and unbounded. We will discuss other aspects such as decidability of such fields. This is joint work with Will Johnson and Erik Walsberg.
URL:https://www.math.ens.psl.eu/evenement/curve-excluding-fields-2/
LOCATION:salle 1016 Sophie Germain
CATEGORIES:Théorie des Modèles et Groupes
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20220315T140000
DTEND;TZID=Europe/Paris:20220315T170000
DTSTAMP:20260418T161447
CREATED:20220305T170139Z
LAST-MODIFIED:20220305T170529Z
UID:15335-1647352800-1647363600@www.math.ens.psl.eu
SUMMARY:Un après-midi de théorie des groupes
DESCRIPTION:Le séminaire sera dans salle W et retransmis sur Zoom : \nZOOM: https://us02web.zoom.us/j/82070470538\nID: 820 7047 0538\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 Marcin Sabok  (McGill University)\, « Hyperfiniteness at\nhyperbolic boundries » \n15.00 – 15.45 Juan Paucar (Jussieu)\, « Coarse embeddings between locally\ncompact groups and quantitative measured equivalence » \n16.00 – 16.45 Josh Frisch (ENS)\, « Characteristic Measures and Minimal\nSubdynamics » \nVous pourrez trouver les résumés sur le site du séminaire:\nhttps://sites.google.com/site/annaerschler/grseminar
URL:https://www.math.ens.psl.eu/evenement/un-apres-midi-de-theorie-des-groupes/
LOCATION:14:00-17:00 Salle W
CATEGORIES:Séminaire de théorie des groupes à l’ENS
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20220309T110000
DTEND;TZID=Europe/Paris:20220309T120000
DTSTAMP:20260418T161447
CREATED:20220218T132131Z
LAST-MODIFIED:20220218T132131Z
UID:15310-1646823600-1646827200@www.math.ens.psl.eu
SUMMARY:Najib Idrissi\, raconte-moi les opérades !
DESCRIPTION:Les opérades sont des objets qui gouvernent des catégories d’algèbres au sens large — par exemple\, les algèbres associatives\, les algèbres commutatives\, ou les algèbres de Lie — qui sont habituellement définies par « opérations génératrices et relations ». Le but de cet exposé est d’introduire la théorie des opérades avec des exemples\, et en particulier l’exemple fondateur des opérades des petits disques. J’expliquerai comment les opérades des petits disques permettent d’obtenir des invariants des variétés de deux façons duales : le calcul des plongements et l’homologie de factorisation.
URL:https://www.math.ens.psl.eu/evenement/najib-idrissi-raconte-moi-les-operades/
LOCATION:En salle W au DMA\, ou sur Zoom
CATEGORIES:Algèbre et géométrie,Séminaire Raconte-moi
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20220301T160000
DTEND;TZID=Europe/Paris:20220301T173000
DTSTAMP:20260418T161447
CREATED:20220223T142159Z
LAST-MODIFIED:20220309T160937Z
UID:15319-1646150400-1646155800@www.math.ens.psl.eu
SUMMARY:Existentially closed measure-preserving actions of free groups
DESCRIPTION:I will discuss a joint work with Alexander Berenstein and Ward Henson\, in which we show that the theory of probability algebras with two automorphisms has a model completion\, which moreover has quantifier elimination and is stable. We also exhibit two non-isomorphic (but approximately isomorphic) models of the model completion.\nMore generally\, we give a sufficient set of conditions for the axiomatizability (in continuous logic) of the existentially closed actions of a free group on a separably categorical\, stable structure.\nI will also mention a number of open questions.
URL:https://www.math.ens.psl.eu/evenement/existentially-closed-measure-preserving-actions-of-free-groups/
LOCATION:Sophie Germain salle 1016.
CATEGORIES:Théorie des Modèles et Groupes
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20220222T160000
DTEND;TZID=Europe/Paris:20220222T173000
DTSTAMP:20260418T161447
CREATED:20220214T184113Z
LAST-MODIFIED:20220214T184113Z
UID:15214-1645545600-1645551000@www.math.ens.psl.eu
SUMMARY:NIPn fields part 2: random hypergraphs and NIPn CHIPS transfer
DESCRIPTION:A core question in the model theory of fields is to understand how combinatorial patterns and algebraic properties interact. The study of NIPn fields\, which can’t express the edge relation of random n-hypergraph\, is linked to henselianity. In this talk\, we use Chernikov and Hils conditions to obtain transfer in some situations\, that is\, under some algebraic assumptions\, it is enough to know that the residue field of a henselian valued field is NIPn in order to known that it is itself NIPn\, and we discuss consequences on hypothetical strictly NIPn fields.
URL:https://www.math.ens.psl.eu/evenement/nipn-fields-part-2-random-hypergraphs-and-nipn-chips-transfer/
LOCATION:salle 1016 Sophie Germain
CATEGORIES:Théorie des Modèles et Groupes
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20220218T154500
DTEND;TZID=Europe/Paris:20220218T171500
DTSTAMP:20260418T161447
CREATED:20220203T145355Z
LAST-MODIFIED:20220211T161457Z
UID:15158-1645199100-1645204500@www.math.ens.psl.eu
SUMMARY:Not Pfaffian
DESCRIPTION:This talk describes the connection between /strong minimality/ of the differential equation satisfied by an complex analytic function and the real and imaginary parts of the function being /Pfaffian/. The talk will not assume the audience knows these notions previously\, and will attempt to motivate why each of them are important notions in various areas. The connection we give\, combined with a theorem of Freitag and Scanlon (2017) provides the answer to a question of Binyamini and Novikov (2017). We also answer a question of Bianconi (2016). We give what seem to be the first examples of functions which are definable in o-minimal expansions of the reals and are differentially algebraic\, but not Pfaffian.
URL:https://www.math.ens.psl.eu/evenement/not-pfaffian/
LOCATION:Zoom
CATEGORIES:Séminaire Géométrie et théorie des modèles
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20220218T140000
DTEND;TZID=Europe/Paris:20220218T153000
DTSTAMP:20260418T161447
CREATED:20220204T103217Z
LAST-MODIFIED:20220204T103815Z
UID:15163-1645192800-1645198200@www.math.ens.psl.eu
SUMMARY:The Kemperman inverse 	problem
DESCRIPTION:Let G be a connected locally compact group with a left Haar measure μ\, and let A\,B ⊆ G be nonempty and compact. Assume further that G is unimodular\, i.e.\, μ is also the right Haar measure; this holds\, e.g.\, when G is compact\, a nilpotent Lie group\, or a semisimple Lie group. In 1964\, Kemperman showed that \nμ(AB) ≥ min {μ(A)+μ(B)\, μ(G)} .\nThe Kemperman inverse problem (proposed by Griesmer\, Kemperman\, and Tao) asks when the equality happens or nearly happens. I will discuss the recent solution of this problem\, highlighting the connections to model theory. (Joint with Jinpeng An\, Yifan Jing\, and Ruixiang Zhang).
URL:https://www.math.ens.psl.eu/evenement/the-kemperman-inverse-problem/
LOCATION:Zoom
CATEGORIES:Séminaire Géométrie et théorie des modèles
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20220216T110000
DTEND;TZID=Europe/Paris:20220216T120000
DTSTAMP:20260418T161447
CREATED:20220214T103226Z
LAST-MODIFIED:20220214T103301Z
UID:15204-1645009200-1645012800@www.math.ens.psl.eu
SUMMARY:Léonard Pille-Schneider\, raconte-moi les espaces hybrides !
DESCRIPTION:Soit X=(X_t) une famille de variétés algébriques complexes paramétrée par le disque épointé\, dont les équations ont une singularité méromorphe en t=0. Le but de cet exposé est d’expliquer comment associer à cette famille un espace dit hybride\, permettant de voir les variétés complexes X_t dégénérer vers l’espace analytique non-archimédien obtenu en interprétant X comme une variété algébrique sur le corps des séries de Laurent. Je donnerai aussi des applications géométriques de cette construction.
URL:https://www.math.ens.psl.eu/evenement/leonard-pille-schneider-raconte-moi-les-espaces-hybrides/
LOCATION:En salle W au DMA\, ou sur Zoom
CATEGORIES:Séminaire Raconte-moi
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20220215T160000
DTEND;TZID=Europe/Paris:20220215T173000
DTSTAMP:20260418T161447
CREATED:20220207T145840Z
LAST-MODIFIED:20220211T161554Z
UID:15168-1644940800-1644946200@www.math.ens.psl.eu
SUMMARY:Groups definable in partial differential fields with an automorphism
DESCRIPTION:This is a joint work with Ronald Bustamante Medina and Zoé Chatzidakis.\nIn this talk we are interested in differential and difference fields from the model-theoretic point of view. A differential field is a field with a set of commuting derivations and a difference-differential field is a differential field equipped with an automorphism which commutes with the derivations.\nCassidy studied definable groups in differentially closed fields\, in particular she studied Zariski dense definable subgroups of simple algebraic groups and showed that they are isomorphic to the rational points of an algebraic group over some definable field. In this talk we study groups definable in existentially closed difference-differential fields. In particular\, we study Zariski dense definable subgroups of simple algebraic groups\, and show an\nanalogue of Phyllis Cassidy’s result for partial differential fields.
URL:https://www.math.ens.psl.eu/evenement/groups-definable-in-partial-differential-fields-with-an-automorphism/
LOCATION:Sophie Germain salle 1016.
CATEGORIES:Théorie des Modèles et Groupes
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20220208T150000
DTEND;TZID=Europe/Paris:20220208T180000
DTSTAMP:20260418T161447
CREATED:20220129T155631Z
LAST-MODIFIED:20220129T160053Z
UID:15136-1644332400-1644343200@www.math.ens.psl.eu
SUMMARY:Un après-midi de sous-groupes aleatoires invariants ou stationaires
DESCRIPTION:ZOOM: https://us02web.zoom.us/j/81548053762\nID: 815 4805 3762\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. \n15.00 – 15.45    Tsachik Gelander (Weizmann Institute)\, « Stationary\nrandom discrete subgroups of semisimple Lie groups » \n16.00 – 16.45     Matthieu Joseph (ENS Lyon)\, « Allosteric actions of\nsurface groups » \n17.15 – 18.00     Yair Hartman (Ben Gurion University)\,\n« Intersectional Invariant Random Subgroups » \nVous pourrez trouver les résumés sur le site du séminaire: \nhttps://sites.google.com/site/annaerschler/grseminar
URL:https://www.math.ens.psl.eu/evenement/un-apres-midi-de-sous-groupes-aleatoires-invariants-ou-stationaires/
LOCATION:Zoom
CATEGORIES:Séminaire de théorie des groupes à l’ENS
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20220202T110000
DTEND;TZID=Europe/Paris:20220202T120000
DTSTAMP:20260418T161447
CREATED:20220214T103412Z
LAST-MODIFIED:20220214T103412Z
UID:15206-1643799600-1643803200@www.math.ens.psl.eu
SUMMARY:Julien Marché\, raconte-moi la topologie quantique et le nombre d'or !
DESCRIPTION:Si la topologie quantique est née des travaux de Jones\, Kauffman et Witten à la fin des années 1980\, on peut lui trouver des racines plus anciennes. En partant des polynômes chromatiques des graphes (Birkhoff 1912)\, revisités par Tutte dans les années 1960\, on va expliquer comment en tirer des représentations des groupes modulaires des surfaces toujours liées au nombre d’or. Parmi elles\, le groupe de l’icosaèdre et l’uniformisation de surfaces trouvées par Hirzebruch.
URL:https://www.math.ens.psl.eu/evenement/julien-marche-raconte-moi-la-topologie-quantique-et-le-nombre-dor/
LOCATION:En salle W au DMA\, ou sur Zoom
CATEGORIES:Séminaire Raconte-moi
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20220121T154000
DTEND;TZID=Europe/Paris:20220121T171000
DTSTAMP:20260418T161447
CREATED:20220106T215337Z
LAST-MODIFIED:20220106T215337Z
UID:15010-1642779600-1642785000@www.math.ens.psl.eu
SUMMARY:Decidability via the tilting correspondence
DESCRIPTION:We discuss new decidability and undecidability results for mixed characteristic henselian fields\, whose proof goes via reduction to positive characteristic. The reduction uses extensively the theory of perfectoid fields and also the earlier Krasner-Kazhdan-Deligne principle. Our main results will be:\n(1) A relative decidability theorem for perfectoid fields. Using this\, we obtain decidability of certain tame fields of mixed characteristic.\n(2) An undecidability result for the asymptotic theory of all finite extensions of ℚ_p (fixed p) with cross-section.\nWe will also discuss a tentative step towards understanding the underlying model theory of arithmetic phenomena in this area\, by presenting a model-theoretic way of seeing the Fontaine-Wintenberger theorem
URL:https://www.math.ens.psl.eu/evenement/decidability-via-the-tilting-correspondence/
LOCATION:En ligne
CATEGORIES:Séminaire Géométrie et théorie des modèles
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20220121T140000
DTEND;TZID=Europe/Paris:20220121T153000
DTSTAMP:20260418T161447
CREATED:20220106T214555Z
LAST-MODIFIED:20220106T215523Z
UID:15001-1642773600-1642779000@www.math.ens.psl.eu
SUMMARY:Hensel minimality and counting in valued fields
DESCRIPTION:Hensel minimality is a new axiomatic framework for doing tame geometry in non-Archimedean fields\, aimed to mimic o-minimality. It is designed to be broadly applicable while having strong consequences. We will give a general overview of the theory of Hensel minimality. Afterwards\, we discuss arithmetic applications to counting rational points on definable sets in valued fields.\nThis is partially joint work with R. Cluckers\, I. Halupczok and S. Rideau-Kikuchi\, and partially with V. Cantoral-Farfan and K. Huu Nguyen.
URL:https://www.math.ens.psl.eu/evenement/hensel-minimality-and-counting-in-valued-fields/
LOCATION:En ligne
CATEGORIES:Séminaire Géométrie et théorie des modèles
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20220119T110000
DTEND;TZID=Europe/Paris:20220119T120000
DTSTAMP:20260418T161447
CREATED:20220117T083010Z
LAST-MODIFIED:20220117T083010Z
UID:15061-1642590000-1642593600@www.math.ens.psl.eu
SUMMARY:François Charles\, raconte-moi la mesure gaussienne sur les réseaux euclidiens !
DESCRIPTION:Je discuterai certaines des nombreuses applications de la mesure gaussienne sur les réseaux euclidiens en mathématiques et en informatique. Dans un deuxième temps\, j’expliquerai comment les mesures gaussiennes apparaissent dans l’étude de certains réseaux de rang infini\, quelles sont les concepts mathématiques qui apparaissent dans cette situation\, et je donnerai des applications arithmétiques.
URL:https://www.math.ens.psl.eu/evenement/francois-charles-raconte-moi-la-mesure-gaussienne-sur-les-reseaux-euclidiens/
LOCATION:En salle W au DMA\, ou sur Zoom
CATEGORIES:Algèbre et géométrie,Séminaire Raconte-moi
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20220111T150000
DTEND;TZID=Europe/Paris:20220111T180000
DTSTAMP:20260418T161447
CREATED:20220109T083122Z
LAST-MODIFIED:20220109T091352Z
UID:15035-1641913200-1641924000@www.math.ens.psl.eu
SUMMARY:Après-midi de moyennabilité
DESCRIPTION:ZOOM: https://us02web.zoom.us/j/88011323267\nID: 880 1132 3267\nMot de passe:\nG est un Graphe de Cayley du groupe libre à 107 générateurs. Quel est le\ndegré de ce graphe? Tapez le numéro à trois chiffres comme un mot de\npasse. \n15.00 – 15.45     Friedrich Martin Schneider (Freiberg)\, « Concentration of\ninvariant means » \n16.00 – 16.45      Eduardo Scarparo (Federal University of Santa\nCatarina)\, « Amenability and unitary representations of groups of dynamical\norigin » \n17.15 – 18.00     Gidi Amir (Bar Ilan)\, « Amenability of quadratic activity\nautomata groups » \nhttps://sites.google.com/site/annaerschler/grseminar
URL:https://www.math.ens.psl.eu/evenement/apres-midi-de-moyennabilite/
LOCATION:Zoom
CATEGORIES:Séminaire de théorie des groupes à l’ENS
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20220105T110000
DTEND;TZID=Europe/Paris:20220105T120000
DTSTAMP:20260418T161447
CREATED:20211226T164951Z
LAST-MODIFIED:20220103T145206Z
UID:14937-1641380400-1641384000@www.math.ens.psl.eu
SUMMARY:Viviane Baladi : billards chaotiques et espaces anisotropes\, le mariage réussi de la carpe et du lapin !
DESCRIPTION:Les espaces de distributions anisotropes sont des outils efficaces pour étudier les propriétés statistiques de dynamiques chaotiques assez régulières\, en reliant ces propriétés au spectre d’un opérateur de type Perron-Frobenius agissant sur ces espaces. Les billards dispersifs (ou billards de Sinai) sont un exemple de dynamique chaotique naturel\, mais très peu régulier : la dynamique est seulement lisse par morceaux\, avec des dérivées non bornées et les « feuilletages dynamiques » sont seulement mesurables. J’expliquerai comment on a pu malgré tout définir et utiliser les espaces anisotropes avec succès dans ce contexte (notamment pour obtenir le mélange exponentiel de la mesure de Liouville du flot billard et construire la mesure maximisant l’entropie de l’application billard). (Travaux avec M. Demers et/ou C. Liverani. J’évoquerai\, si le temps le permet\, des travaux en cours de membres de mon groupe à Jussieu.)
URL:https://www.math.ens.psl.eu/evenement/viviane-baladi-billards-chaotiques-et-espaces-anisotropes-le-mariage-reussi-de-la-carpe-et-du-lapin/
LOCATION:En hybride\, 45 rue d'Ulm\, Paris\, 75005
CATEGORIES:Séminaire Raconte-moi
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20211202T090000
DTEND;TZID=Europe/Paris:20211202T120000
DTSTAMP:20260418T161447
CREATED:20211202T080000Z
LAST-MODIFIED:20211104T114114Z
UID:14115-1638435600-1638446400@www.math.ens.psl.eu
SUMMARY:On dp-finite fields
DESCRIPTION:Shelah’s conjecture predicts that any infinite NIP field iseither separably closed\, real closed or admits a non-trivial henselianvaluation. Recently\, Johnson proved that Shelah’s conjecture holds forfields of finite dp-rank\, also known as dp-finite fields. The aim of these two talks is to give an introduction to dp-rank in some algebraic structures and an overview of Johnson’s work.In the first talk\, we define dp-rank (which is a notion of rank in NIP theories) and give examples of dp-finite structures. In particular\, we discuss the dp-rank of ordered abelian groups and use them to construct multitude of examples of dp-finite fields. We also prove that every dp-finite field is perfect and sketch a proof that any valued field of dp-rank 1 is henselian.In the second talk\, we give an overview of Johnson’s proof that everyinfinite dp-finite field is either algebraically closed\, real closed oradmits a non-trivial henselian valuation. Crucially\, this relies on the notion of a W-topology\, a natural generalization of topologies arising from valuations\, and the construction of a definable W-topology on asufficiently saturated unstable dp-finite field.
URL:https://www.math.ens.psl.eu/evenement/on-dp-finite-fields-2/
LOCATION:Zoom
CATEGORIES:Séminaire Géométrie et théorie des modèles
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20211110T110000
DTEND;TZID=Europe/Paris:20211110T120000
DTSTAMP:20260418T161447
CREATED:20211109T095100Z
LAST-MODIFIED:20211109T151636Z
UID:14454-1636542000-1636545600@www.math.ens.psl.eu
SUMMARY:« Joshua Frisch\, raconte-moi la moyennabilité forte ! »
DESCRIPTION:A topological dynamical system (i.e. a group acting by homeomorphisms on a compact metric space) is said to be proximal if for any two points p and q we can simultaneously « squish them together ». A group is strongly amenable if every proximal dynamical system has a fixed point. In this talk I will give an introduction to proximal actions\, strong amenability and discuss connections with other group theoretic properties. No prior knowledge of topological dynamics or amenability will be assumed. \nEn salle W au DMA\, ou sur Zoom (réunion 997 7829 7310\, mot de passe abc123).
URL:https://www.math.ens.psl.eu/evenement/joshua-frisch-raconte-moi-la-moyennabilite-forte/
LOCATION:En hybride\, 45 rue d'Ulm\, Paris\, 75005
CATEGORIES:Séminaire Raconte-moi
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20211011T140000
DTEND;TZID=Europe/Paris:20211011T170000
DTSTAMP:20260418T161447
CREATED:20211011T120000Z
LAST-MODIFIED:20211104T133512Z
UID:14131-1633960800-1633971600@www.math.ens.psl.eu
SUMMARY:Après-midi de théorie de groupes
DESCRIPTION:14.00 – 14.45 François Le Maître (Université Paris Diderot -Paris VII)\, Reconstruction for Boolean measure-preserving actions of full groups and applications15.00 – 15.45 Romain Tessera (Université Paris Diderot -Paris VII)\, Coarse geometry meets measured group theory16.00 – 16.45 Pierre Fima (Université Paris Diderot -Paris VII)\, Highly transitive groups among groups acting on trees
URL:https://www.math.ens.psl.eu/evenement/apres-midi-de-theorie-de-groupes-17/
LOCATION:Salle W (DMA ENS)
CATEGORIES:Séminaire de théorie des groupes à l’ENS
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20210618T164000
DTEND;TZID=Europe/Paris:20210618T180000
DTSTAMP:20260418T161447
CREATED:20210618T144000Z
LAST-MODIFIED:20211104T140211Z
UID:8578-1624034400-1624039200@www.math.ens.psl.eu
SUMMARY:Real perspectives on monomialization.
DESCRIPTION:I will discuss recent work in collaboration with Edward Bierstone on transformation of a mapping to monomial form (with respect to local coordinates) by simple modifications of the source and target. Our techniques apply in a uniform way to the algebraic and analytic categories\, as well as to classes of infinitely differentiable real functions that are quasianalytic or definable in an o-minimal structure. Our results in the real cases are best possible. The talk will focus on real phenomena and on an application to quantifier elimination of certain o-minimal polynomially bounded structure.
URL:https://www.math.ens.psl.eu/evenement/real-perspectives-on-monomialization/
LOCATION:Zoom
CATEGORIES:Séminaire Géométrie et théorie des modèles
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20210618T150000
DTEND;TZID=Europe/Paris:20210618T162000
DTSTAMP:20260418T161447
CREATED:20210618T130000Z
LAST-MODIFIED:20211025T103955Z
UID:8577-1624028400-1624033200@www.math.ens.psl.eu
SUMMARY:Monadically NIP ordered graphs and bounded twin-width
DESCRIPTION:An open problem in theoretical computer science asks to characterize tameness for hereditary classes of finite structures. The notion of bounded twin-width was proposed and studied recently by Bonnet\, Geniet\, Kim\, Thommasé and Watrignant. Classes of graphs of bounded twin-width have many desirable properties. In particular\, they are monadically NIP (remain NIP after naming arbitrary unary predicates). In joint work with Szymon Torunczyk we show the converse for classes of ordered graphs. We then obtain a very clear dichotomy between tame (slow growth\, monadically NIP\, algorithmically simple …) and wild hereditary classes of ordered graphs. Those results were also obtained by Bonnet\, Giocanti\, Ossona de Mendez and Thomassé. In this talk\, I will focus on the model theoretic input.
URL:https://www.math.ens.psl.eu/evenement/monadically-nip-ordered-graphs-and-bounded-twin-width/
LOCATION:Zoom
CATEGORIES:Séminaire Géométrie et théorie des modèles
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20210521T103000
DTEND;TZID=Europe/Paris:20210521T115000
DTSTAMP:20260418T161447
CREATED:20210521T083000Z
LAST-MODIFIED:20211025T103953Z
UID:8576-1621593000-1621597800@www.math.ens.psl.eu
SUMMARY:Pseudo-T-closed fields\,  approximations and NTP2
DESCRIPTION:Joint work with Samaria MontenegroThe striking resemblance between the behaviour of pseudo-algebraically closed\, pseudo real closed and pseudo p-adically fields has lead to numerous attempts at describing their properties in a unified manner. In this talk I will present another of these attempts: the class of pseudo-T-closed fields\, where T is an enriched theory of fields. These fields verify a “local-global” principle with respect to models of T for the existence of points on varieties. Although it very much resembles previous such attempts\, our approach is more model theoretic in flavour\, both in its presentation and in the results we aim for.The first result I would like to present is an approximation result\, generalising a result of Kollar on PAC fields\, respectively Johnson on henselian fields. This result can be rephrased as the fact that existential closeness in certain topological enrichments come for free from existential closeness as a field. The second result is a (model theoretic) classification result for bounded pseudo-T-closed fields\, in the guise of the computation of their burden. One of the striking consequence of these two results is that a bounded perfect PAC field with n independent valuations has burden n and\, in particular\, is NTP2.
URL:https://www.math.ens.psl.eu/evenement/pseudo-t-closed-fields-approximations-and-ntp2/
LOCATION:Zoom
CATEGORIES:Séminaire Géométrie et théorie des modèles
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20210521T090000
DTEND;TZID=Europe/Paris:20210521T102000
DTSTAMP:20260418T161447
CREATED:20210521T070000Z
LAST-MODIFIED:20211104T140240Z
UID:8575-1621587600-1621592400@www.math.ens.psl.eu
SUMMARY:Curve-excluding fields
DESCRIPTION:Let T be the theory of fields K of characteristic 0 such that the equation x^4 + y^4 = 1 has only four solutions in K. We show that T has a model companion. More generally\, if K_0 is a field of characteristic 0 and C is a curve (affine or projective) of genus ≥ 2 with C(K_0) = ∅\, then there is a model companion CXF of the theory of fields K extending K_0 with C(K) = ∅.\nWe can use this theory to construct a field K with an interesting combination of properties. On the model-theoretic side\, the theory of K is complete\, decidable\, model-complete\, and algebraically bounded\, and K is a “geometric structure” in the sense of Hrushovski and Pillay. Additionally\, some classification-theoretic properties might hold in K. On the field-theoretic side\, K is non-large—there is a smooth curve C such that C(K) is finite and non-empty. This is unusual; the vast majority of model-theoretically tractable fields are large or finite. On the other hand\, K is “virtually large”—it has a finite extension which is large. In fact\, every proper algebraic extension of K is pseudo algebraically closed (PAC). The absolute Galois group of K is an ω-free profinite group. This negatively answers a question of Junker and Koenigsmann (is every model-complete infinite field large?) and a question of Macintyre (does every model-complete field have a small Galois group?).\nThis is based on joint work with Erik Walsberg and Vincent Ye.
URL:https://www.math.ens.psl.eu/evenement/curve-excluding-fields/
LOCATION:Zoom
CATEGORIES:Séminaire Géométrie et théorie des modèles
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20210423T163000
DTEND;TZID=Europe/Paris:20210423T175000
DTSTAMP:20260418T161447
CREATED:20210423T143000Z
LAST-MODIFIED:20211025T103942Z
UID:8573-1619195400-1619200200@www.math.ens.psl.eu
SUMMARY:Recognizing groups and fields in Erdős geometry and model theory
DESCRIPTION:Assume that Q is a relation on R^s of arity s definable in an o-minimal expansion of R. I will discuss how certain extremal asymptotic behaviors of the sizes of the intersections of Q with finite n  × … × n grids\, for growing n\, can only occur if Q is closely connected to a certain algebraic structure.On the one hand\, if the projection of Q onto any s-1 coordinates is finite-to-one but Q has maximal size intersections with some grids (of size >n^(s-1 – ε))\, then Q restricted to some open set is\, up to coordinatewise homeomorphisms\, of the form x_1+…+x_s=0. This is a special case of the recent generalization of the Elekes-Szabó theorem to any arity and dimension in which general abelian Lie groups arise (joint work with Kobi Peterzil and Sergei Starchenko).On the other hand\, if Q omits a finite complete s-partite hypergraph but can intersect finite grids in more that than n^(s-1 + ε) points\, then the real field can be definably recovered from Q (joint work with Abdul Basit\, Sergei Starchenko\, Terence Tao and Chieu-Minh Tran).I will explain how these results are connected to the model-theoretic trichotomy principle and discuss variants for higher dimensions\, and for stable structures with distal expansions.
URL:https://www.math.ens.psl.eu/evenement/recognizing-groups-and-fields-in-erds-geometry-and-model-theory/
LOCATION:Zoom
CATEGORIES:Séminaire Géométrie et théorie des modèles
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20210423T150000
DTEND;TZID=Europe/Paris:20210423T162000
DTSTAMP:20260418T161447
CREATED:20210423T130000Z
LAST-MODIFIED:20211104T140311Z
UID:8574-1619190000-1619194800@www.math.ens.psl.eu
SUMMARY:VC-dimension in model theory\, discrete geometry\, and combinatorics
DESCRIPTION:In statistical learning theory\, the notion of VC-dimension was developed by Vapnik and Chervonenkis in the context of approximating probabilities of events by the relative frequency of random test points. This notion has been widely used in combinatorics and computer science\, and is also directly connected to model theory through the study of NIP theories. This talk will start with an overview of VC-dimension\, with examples motivated by discrete geometry and additive combinatorics. I will then present several model theoretic applications of VC-dimension. The selection of topics will focus on the use of finitely approximable Keisler measures to analyze the structure of algebraic and combinatorial objects with bounded VC-dimension.
URL:https://www.math.ens.psl.eu/evenement/vc-dimension-in-model-theory-discrete-geometry-and-combinatorics/
LOCATION:Zoom
CATEGORIES:Séminaire Géométrie et théorie des modèles
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20210330T140000
DTEND;TZID=Europe/Paris:20210330T170000
DTSTAMP:20260418T161447
CREATED:20210330T120000Z
LAST-MODIFIED:20211108T205327Z
UID:8572-1617112800-1617123600@www.math.ens.psl.eu
SUMMARY:Après-midi de marches aléatoires et des groupes moyennables
DESCRIPTION:Zoom: https://us02web.zoom.us/j/85927181837\nMot de passe: G est un Graphe de Cayley du groupe libre à 107 générateurs. Quel est le degré de ce graphe?\nTapez le numéro à trois chiffres comme un mot de passe. \n\n14.00 – 14.45 Hanna Oppelmayer\, Random walks on dense subgroups of totally disconnected locally compact groups\n15.00 – 15.45 Georgii Veprev\, Non-existence of a universal zero entropy system for non-periodic amenable group actions\n16.15 – 17.00 Paul-Henry Leemann\, De Bruijn graphs\, spider web graphs and Lamplighter groups
URL:https://www.math.ens.psl.eu/evenement/apres-midi-de-marches-aleatoires-et-des-groupes-moyennables/
LOCATION:Zoom: https://us02web.zoom.us/j/85927181837
CATEGORIES:Séminaire de théorie des groupes à l’ENS
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20210326T163000
DTEND;TZID=Europe/Paris:20210326T175000
DTSTAMP:20260418T161447
CREATED:20210326T153000Z
LAST-MODIFIED:20211025T103938Z
UID:8571-1616776200-1616781000@www.math.ens.psl.eu
SUMMARY:Linearization procedures in the semi-minimal analysis of algebraic differential equations
DESCRIPTION:It is well-known that certain algebraic differential equations restrain in an essential way the algebraic relations that their solutions share. For example\, the solutions of the first equation of Painlevé y » = 6y^2 + t are “new” transcendental functions of order two which whenever distinct are algebraically independent (together with their derivatives).I will first describe an account of such phenomena using the language of geometric stability theory in a differentially closed field. I will then explain how linearization procedures and geometric stability theory fit together to study such transcendence results in practice.
URL:https://www.math.ens.psl.eu/evenement/linearization-procedures-in-the-semi-minimal-analysis-of-algebraic-differential-equations/
LOCATION:Zoom
CATEGORIES:Séminaire Géométrie et théorie des modèles
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20210326T150000
DTEND;TZID=Europe/Paris:20210326T162000
DTSTAMP:20260418T161447
CREATED:20210326T140000Z
LAST-MODIFIED:20211104T140339Z
UID:8570-1616770800-1616775600@www.math.ens.psl.eu
SUMMARY:Effective isotrivial Mordell-Lang in positive characteristic
DESCRIPTION:The Mordell-Lang conjecture (now a theorem\, proved by Faltings\, Vojta\, McQuillan\,…) asserts that if G is a semiabelian variety G defined over an algebraically closed field of characteristic zero\, X is a subvariety of G\, and Γ is a finite rank subgroup of G\, then X ∩ Γ is a finite union of cosets of Γ. In positive characteristic\, the naive translation of this theorem does not hold\, however Hrushovski\, using model theoretic techniques\, showed that in some sense all counterexamples arise from semiabelian varieties defined over finite fields (the isotrivial case). This was later refined by Moosa and Scanlon\, who showed in the isotrivial case that the intersection of a subvariety of a semiabelian variety G with a finitely generated subgroup Γ of G that is invariant under the Frobenius endomorphism F: G → G is a finite union of sets of the form S+A\, where A is a subgroup of Γ and S is a sum of orbits under the map F. We show how how one can use finite-state automata to give a concrete description of these intersections Γ ∩ X in the isotrivial setting\, by constructing a finite machine that identifies all points in the intersection. In particular\, this allows us to give decision procedures for answering questions such as: is X ∩ Γ empty? finite? does it contain a coset of an infinite subgroup? In addition\, we are able to read off coarse asymptotic estimates for the number of points of height ≤ H in the intersection from the machine. This is joint work with Dragos Ghioca and Rahim Moosa.
URL:https://www.math.ens.psl.eu/evenement/effective-isotrivial-mordell-lang-in-positive-characteristic/
LOCATION:Zoom
CATEGORIES:Séminaire Géométrie et théorie des modèles
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20210119T160000
DTEND;TZID=Europe/Paris:20210119T190000
DTSTAMP:20260418T161447
CREATED:20210119T150000Z
LAST-MODIFIED:20211104T134537Z
UID:8569-1611072000-1611082800@www.math.ens.psl.eu
SUMMARY:Après-midi de théorie de groupes
DESCRIPTION:Zoom: https://us02web.zoom.us/j/84778703586Mot de passe: G 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.16.00-16.45 Igor Pak\, Cogrowth sequences in groups and graphs17.00-17.45 Behrang Forghani\, Boundary Preserving Transformations18.15-19.00 Mehrdad Kalantar\, On weak containment properties of quasi-regular representations of stabilizer subgroups of boundary actions
URL:https://www.math.ens.psl.eu/evenement/apres-midi-de-theorie-de-groupes-12/
LOCATION:Zoom: https://us02web.zoom.us/j/84778703586
CATEGORIES:Séminaire de théorie des groupes à l’ENS
END:VEVENT
END:VCALENDAR