BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//Département de mathématiques et applications - ECPv6.2.2//NONSGML v1.0//EN
CALSCALE:GREGORIAN
METHOD:PUBLISH
X-ORIGINAL-URL:https://www.math.ens.psl.eu
X-WR-CALDESC:évènements pour Département de mathématiques et applications
REFRESH-INTERVAL;VALUE=DURATION:PT1H
X-Robots-Tag:noindex
X-PUBLISHED-TTL:PT1H
BEGIN:VTIMEZONE
TZID:Europe/Paris
BEGIN:DAYLIGHT
TZOFFSETFROM:+0100
TZOFFSETTO:+0200
TZNAME:CEST
DTSTART:20260329T010000
END:DAYLIGHT
BEGIN:STANDARD
TZOFFSETFROM:+0200
TZOFFSETTO:+0100
TZNAME:CET
DTSTART:20261025T010000
END:STANDARD
END:VTIMEZONE
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20260408T140000
DTEND;TZID=Europe/Paris:20260408T170000
DTSTAMP:20260430T192631
CREATED:20260403T082003Z
LAST-MODIFIED:20260403T082112Z
UID:21215-1775656800-1775667600@www.math.ens.psl.eu
SUMMARY:Un après-midi de théorie des groupes - Adrien Boyer\, Julien Marché\, Greg Patchell
DESCRIPTION:14:00-14:45 Adrien Boyer (IMJ-PRG): Property RD and Boundary Representations for A2 Buildings  \n\n\n\n15:00-15:45 Julien Marché (ENS – PSL): Action of endomorphisms of free groups on their SL_2-character varieties  \n\n\n\n16:15-17:00 Greg Patchell (University of Oxford): Selfless Inclusions of C*-Algebras and Quantum Groups \n\n\n\n \n\n\n\nAbstracts: \n\n\n\nGreg Patchell: Selfless Inclusions of C*-Algebras and Quantum Groups \n\n\n\nRecently\, strong asymptotic freeness\, or selflessness\, in C-algebras has emerged as a powerful technique to prove important regularity properties including simplicity\, unique trace\, stable rank 1\, and strict comparison. In particular\, in Fall 2024\, Amrutam\, Gao\, Kunnwalkam Elayavalli\, and I showed that the reduced group C-algebras of all hyperbolic groups with trivial finite radical are selfless\, which resolved the open problem of strict comparison for the reduced group C-algebra of the free group on two generators. Since then\, our result has been expanded to include a much larger class of groups. Work has also begun on isolating selflessness for C-algebras not arising from groups\, including the result of Hayes\, Kunnawalkam Elayavalli\, and Robert on selflessness of the reduced free product of a large class of C-algebras (see also Flores-Klisse-Ó Cobhthaigh-Pagliero). I will introduce the general notion of a selfless inclusion of C-algebras\, with which we will see the selflessness of the reduced unitary compact matrix quantum groups. This work is joint with Ben Hayes\, Srivatsav Kunnawalkam Elayavalli\, and Leonel Robert. \n\n\n\n \n\n\n\nAdrien Boyer: Property RD and Boundary Representations for A2 Buildings \n\n\n\nI will discuss an approach based on boundary representations to prove property RD for discrete groups acting properly and cocompactly on affine buildings of type A2. This result is due to Robertson\, Ramagge\, and Steger in the late 1990s. I will emphasize geometric arguments involving the Furstenberg boundary that can be used to establish property RD\, in particular the notion of “foldings” or “confluences\,” as suggested by V. Kaimanovich. Along the way\, I will also mention a conjecture of Robertson\, Ramagge\, and Steger concerning the optimal bound\, and propose a possible approach toward resolving it. If time permits\, I will also explain what happens in the C2 case. This is joint work with Thang Nguyen. \n\n\n\n \n\n\n\nJulien Marché: Action of endomorphisms of free groups on their SL_2-character varieties. \n\n\n\nLet phi : F_n->F_n be an endomorphism and let phi^* denote its action on the character variety X_n=Hom(F_n\,SL_2(C))/SL_2(C). Cantat-Gelander-Souto raised the question whether phi^* is an automorphism of the affine variety X_n if and only if phi is an automorphism of F_n. I will describe work in progress which relates directly this question to the action of endomorphisms on the outer space CV_n through a compactification of X_n with special valuations.
URL:https://www.math.ens.psl.eu/evenement/un-apres-midi-de-theorie-des-groupes-adrien-boyer-julien-marche-greg-patchell/
LOCATION:Salle W
CATEGORIES:Séminaire de théorie des groupes à l’ENS
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20260506T110000
DTEND;TZID=Europe/Paris:20260506T120000
DTSTAMP:20260430T192631
CREATED:20260430T081744Z
LAST-MODIFIED:20260430T081744Z
UID:21515-1778065200-1778068800@www.math.ens.psl.eu
SUMMARY:Matteo Tamiozzo\, raconte-moi les groupes de Golod-Shafarevich !
DESCRIPTION:Les groupes de Golod-Shafarevich\, découverts dans les années 60\, ont trouvé des applications en théorie des nombres\, théorie des groupes et topologie. Dans cet exposé j’introduirai ces groupes\, et j’expliquerai le rôle qu’ils jouent dans l’étude des groupes fondamentaux des anneaux des entiers des corps de nombres et des variétés de dimension 3.
URL:https://www.math.ens.psl.eu/evenement/matteo-tamiozzo-raconte-moi-les-groupes-de-golod-shafarevich/
LOCATION:Salle W toits du DMA
CATEGORIES:Algèbre et géométrie,Séminaire Raconte-moi
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20260506T140000
DTEND;TZID=Europe/Paris:20260506T170000
DTSTAMP:20260430T192631
CREATED:20260430T080415Z
LAST-MODIFIED:20260430T081156Z
UID:21503-1778076000-1778086800@www.math.ens.psl.eu
SUMMARY:Un après-midi de théorie des groupes - Elia Fioravanti\, Enrico Le Donne\, Catherine Pfaff
DESCRIPTION:14.00 – 14.45   Elia Fioravanti (KIT – Karlsruhe) « Generators for automorphisms of special groups »15.00 – 15.45   Enrico Le Donne (University of Fribourg) « Asymptotic geometry of Riemannian nilpotent groups »16.15 – 17.00   Catherine Pfaff (Queen’s University) « A « cubist » decomposition of the Handel-Mosher axis bundle » \n\n\n\n \n\n\n\nAbstracts: \n\n\n\nElia Fioravanti: Generators for automorphisms of special groups  \n\n\n\nGiven a family F of finitely generated groups\, do all groups in F have “tame” automorphisms\, or can there be “wild” examples? More concretely\, is Out(G) finitely generated for all groups G in the family F? Rips and Sela showed in the 90s that Out(G) is finitely generated for all Gromov-hyperbolic groups G\, while Baues and Grunewald showed in the 00s that Out(G) is arithmetic over Q (and hence finitely generated) for all virtually polycyclic groups G. This essentially exhausts our limited understanding of general phenomena of this kind\, with the structure of automorphisms of non-positively curved groups remaining a fundamental open problem. I will discuss the recent result that Out(G) is finitely generated for all (cocompact) special groups of Haglund and Wise. This is already new for most finite-index subgroups of right-angled Artin and Coxeter groups. \n\n\n\nEnrico Le Donne: Asymptotic geometry of Riemannian nilpotent groups. \n\n\n\nAsymptotic cones of Riemannian nilpotent Lie groups are Carnot groups. The volume of balls in Carnot groups grows exactly as a power of the radius. Heuristically\, the better the asymptotic cone approximates a Riemannian group\, the closer the volume growth approaches a polynomial growth. I will discuss several results obtained over the last few years in collaboration with Breuillard\, Nalon\, Nicolussi Golo\, and Ryoo. \n\n\n\nCatherine Pfaff: A “cubist” decomposition of the Handel-Mosher axis bundle \n\n\n\nA hyperbolic isometry acts on the compactified hyperbolic plane with North-South dynamics and a single invariant axis. The same is true for a pseudo-Anosov mapping class acting on a Teichmuller space and other hyperbolic-like settings. However\, while a fully irreducible free group outer automorphism acts on compactified Outer space with North-South dynamics\, there can be many axes for a single fully irreducible φ ∈ Out(F_r). With this in mind\, Handel and Mosher define the axis bundle for a fully irreducible φ ∈ Out(F_r). And then Handel-Mosher and Bridson-Vogtmann ask about the geometry of the axis bundle. In joint work with Chi Cheuk Tsang\, we show that the axis bundle of a nongeometric fully irreducible outer automorphism admits a canonical “cubist” decomposition into branched cubes that fit together with special combinatorics. From this structure\, we locate a canonical finite collection of periodic fold lines in each axis bundle. This can be considered as an analogue of results of Hamenstadt and Agol from the surface setting\, which state that the set of trivalent train tracks carrying the unstable lamination of a pseudo-Anosov map can be given the structure of a CAT(0) cube complex\, and that there is a canonical periodic fold line in this cube complex. Our “cubist” decomposition also gives a “hands on” solution to the fully irreducible
URL:https://www.math.ens.psl.eu/evenement/un-apres-midi-de-theorie-des-groupes-elia-fioravanti-enrico-le-donne-catherine-pfaff/
LOCATION:Salle W
CATEGORIES:Séminaire de théorie des groupes à l’ENS
END:VEVENT
END:VCALENDAR