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CALSCALE:GREGORIAN
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BEGIN:VEVENT
SUMMARY:Koji Fujiwara (Kyoto)\, Macarena Arenas (Cambridge)\, Indira Chatt
 erji (Nice)
DTSTART:20201027T080000Z
DTEND:20201027T110000Z
DTSTAMP:20260422T225826Z
UID:GroupTheoryENS/1
DESCRIPTION:Title: <a href="https://researchseminars.org/talk/GroupTheoryE
 NS/1/">A group theory morning</a>\nby Koji Fujiwara (Kyoto)\, Macarena Are
 nas (Cambridge)\, Indira Chatterji (Nice) as part of ENS group theory semi
 nar\n\n\nAbstract\n09.00-09.45 Koji Fujiwara (Kyoto) "The rates of growth 
 in a hyperbolic group"\n\n10.00-10.45 Macarena Arenas (Cambridge) "Linear 
 isoperimetric functions for surfaces in hyperbolic groups"\n\nOne of the m
 ain characterisations of word-hyperbolic groups is that they are the group
 s with a linear isoperimetric function. That is\, for\na compact 2-complex
  X\, the hyperbolicity of its fundamental group is equivalent to the exist
 ence of a linear isoperimetric function for\ndisc diagrams D -->X. It is l
 ikewise known that hyperbolic groups have a linear annular\nisoperimetric 
 function and a linear homological isoperimetric function. I will tell you 
 a bit about these isoperimetric functions\nand a generalisation to all hom
 otopy types of surface diagrams. This is joint work with Dani Wise.\n\n\n1
 1.15-12.00 Indira Chatterji (Nice) "Tangent bundles on hyperbolic spaces a
 nd proper actions on Lp spaces".\n\nI will define a notion of a negatively
  curved tangent bundle of a metric measured space\, and relate that notion
  to proper actions on Lp spaces. I will discuss hyperbolic spaces as examp
 les.\n
LOCATION:https://researchseminars.org/talk/GroupTheoryENS/1/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Alessandro Sisto (Heriot-Watt)\, Thomas Haettel (Montpellier)\, Ma
 rk Hagen (Bristol)
DTSTART:20201124T130000Z
DTEND:20201124T160000Z
DTSTAMP:20260422T225826Z
UID:GroupTheoryENS/2
DESCRIPTION:Title: <a href="https://researchseminars.org/talk/GroupTheoryE
 NS/2/">A group theory afternoon</a>\nby Alessandro Sisto (Heriot-Watt)\, T
 homas Haettel (Montpellier)\, Mark Hagen (Bristol) as part of ENS group th
 eory seminar\n\n\nAbstract\n14.00-14.45 Alessandro Sisto (Heriot-Watt)\n\n
 15.00-15.45 Thomas Haettel ( Montpellier)\n\n16.15-17.00 Mark Hagen ( Bris
 tol)\n\n\nAlessandro Sisto "Cubulation of hulls and bicombings"\n\nIt is w
 ell-known that the quasi-convex hull of finitely many points in a\nhyperbo
 lic space is quasi-isometric to a tree. I will discuss an\nanalogous fact 
 in the context of hierarchically hyperbolic spaces\, a\nlarge class of spa
 ces and groups including mapping class groups\,\nTeichmueller space\, righ
 t-angled Artin and Coxeter groups\, and many\nothers. In this context\, th
 e approximating tree is replaced by a CAT(0)\ncube complex. I will also br
 iefly discuss applications\, including how\nthis can be used to construct 
 bicombings.\nBased on joint works with Behrstock-Hagen and Durham-Minsky.\
 n\nThomas Haettel "The coarse Helly property\, hierarchical hyperbolicity 
 and semihyperbolicity"\n\nFor any hierarchical hyperbolic group\, and in p
 articular any mapping\nclass group\, we define a new metric that satisfies
  a coarse Helly\nproperty. This enables us to deduce that the group is sem
 ihyperbolic\,\ni.e. that it admits a bounded quasigeodesic bicombing\, and
  also that\nit has finitely many conjugacy classes of finite subgroups. Th
 is has\nseveral other consequences for the group. This is a joint work wit
 h\nNima Hoda and Harry Petyt.\n\n\n\nMark Hagen "Wallspaces\, the Behrstoc
 k inequality\, and l_1 metrics on\nasymptotic cones"\n\nFrom its hyperplan
 es\, one can always characterise a CAT(0)\ncube complex as the subset of s
 ome (often infinite) cube consisting of\nthe solutions to a system of "con
 sistency" conditions.  Analogously\, a\nhierarchically hyperbolic space (H
 HS) can be coarsely characterised as a\nsubset of a product of Gromov-hype
 rbolic spaces consisting of the\n"solutions" to a system of coarse consist
 ency conditions.\nHHSes are a common generalisation of hyperbolic spaces\,
  mapping class\ngroups\, Teichmuller space\, and right-angled Artin/Coxete
 r groups.  The\noriginal motivation for defining HHSes was to provide a un
 ified\nframework for studying the large-scale properties of examples like 
 these.\nSo\, it is natural to ask about the structure of asymptotic cones 
 of\nhierarchically hyperbolic spaces.\nMotivated by the above characterisa
 tion of a CAT(0) cube complex\, we\nintroduce the notion of an R-cubing.  
 This is a space that can be\nobtained from a product of R-trees\, with the
  l_1 metric\, as a solution\nset of a similar set of consistency condition
 s. R-cubings are therefore\na common generalisation of R-trees and (finite
 -dimensional) CAT(0) cube\ncomplexes.  R-cubings are median spaces with ex
 tra structure\, in much\nthe same way that HHSes are coarse median spaces 
 with extra structure.\nThe main result in this talk says that every asympt
 otic cone of a\nhierarchically hyperbolic space is bilipschitz equivalent 
 to an\nR-cubing.  This strengthens a theorem of Behrstock-Drutu-Sapir abou
 t\nasymptotic cones of mapping class groups.  Time permitting\, I will tal
 k\nabout an application of this result which is still in progress\, namely
 \nuniqueness of asymptotic cones of various hierarchically hyperbolic\ngro
 ups\, including mapping class groups and right-angled Artin groups.\nThis 
 is joint work with Montse Casals-Ruiz and Ilya Kazachkov.\n
LOCATION:https://researchseminars.org/talk/GroupTheoryENS/2/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Robert Young (NYY Courant and IAS Princeton)\, Matei Coiculescu (B
 rown University)\, Richard Schwartz (Brown University  and IAS Princeton)
DTSTART:20201208T140000Z
DTEND:20201208T170000Z
DTSTAMP:20260422T225826Z
UID:GroupTheoryENS/3
DESCRIPTION:Title: <a href="https://researchseminars.org/talk/GroupTheoryE
 NS/3/">A group theory afternoon</a>\nby Robert Young (NYY Courant and IAS 
 Princeton)\, Matei Coiculescu (Brown University)\, Richard Schwartz (Brown
  University  and IAS Princeton) as part of ENS group theory seminar\n\n\nA
 bstract\nRobert Young\,  "Hölder maps to the Heisenberg group"\n\nIn this
  talk\, we construct Hölder maps to the Heisenberg group H\, answering a 
 question of Gromov. Pansu and Gromov observed that any surface embedded in
  H has Hausdorff dimension at least 3\, so there is no α-Hölder embeddin
 g of a surface into H when α > 2/3. Züst improved this result to show th
 at when α > 2/3\, any α-Hölder map from a simply-connected Riemannian m
 anifold to H factors through a metric tree. We use new techniques for cons
 tructing self-similar extensions to show that any continuous map to H can 
 be approximated by a (2/3 - ε)-Hölder map. This is joint work with Stefa
 n Wenger.\n\n\nMatei Coiculescu\, "The Spheres of Sol"\n\nSol\, one of the
  eight Thurston geometries\, is a solvable three-dimensional Lie group equ
 ipped with a canonical left invariant metric. Sol has sectional curvature 
 of both signs and is not rotationally symmetric\, which complicates the st
 udy of its Riemannian geometry.\nOur main result is a characterization of 
 the cut locus of Sol\, which implies as a corollary that the metric sphere
 s in Sol are topological spheres. \nThis is joint work with Richard Schwar
 tz".\n\n\nRichard Schwartz\,  "The areas of metric spheres in Sol"\n\nThis
  is a sequel talk\, following Matei Coiculescu's talk about our joint work
  characterizing the cut locus of the identity in Sol.\nIn this talk\, I wi
 ll explain my result that the area of a metric sphere of radius r in Sol i
 s at most Ce^r for some uniform constant C.  That is\,\nup to constants\, 
 the sphere of radius r in Sol has the same area as the hyperbolic disk of 
 radius r.\n
LOCATION:https://researchseminars.org/talk/GroupTheoryENS/3/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Igor Pak (UCLA)\, Behrang Forghani (the College of Charleston)\, M
 ehrdad Kalantar  (University of Houston)
DTSTART:20210119T150000Z
DTEND:20210119T180000Z
DTSTAMP:20260422T225826Z
UID:GroupTheoryENS/4
DESCRIPTION:Title: <a href="https://researchseminars.org/talk/GroupTheoryE
 NS/4/">A group theory afternoon</a>\nby Igor Pak (UCLA)\, Behrang Forghani
  (the College of Charleston)\, Mehrdad Kalantar  (University of Houston) a
 s part of ENS group theory seminar\n\n\nAbstract\nIgor Pak\, "Cogrowth seq
 uences in groups and graphs"\n\nLet G  be a finitely generated group with 
 generating set S.  We study the cogrowth sequence  {a_n(G\,S)}\, which cou
 nts the number of words of length n over the alphabet S that are equal to 
 1 in G.  I will survey recent asymptotic and analytic results on the cogro
 wth sequence\, motivated by both combinatorial and algebraic applications.
   I will then present our recent work with Kassabov on spectral radii of C
 ayley graphs\, which are also governed by the asymptotics of cogrowth sequ
 ences. \n\n\nBehrang Forghani\, "Boundary Preserving Transformations"\n\nT
 his talk concerns the situations when the Poisson boundaries of different 
 random walks on the same group coincide. In some special cases\, Furstenbe
 rg and Willis addressed this question. However\, the scopes of their const
 ructions are limited. I will show how randomized stopping times can constr
 uct measures that preserve Poisson boundaries and discuss their applicatio
 ns regarding the Poisson boundary identification problem. This talk is bas
 ed on joint work with Kaimanovich.\n\nMehrdad Kalantar\, "On weak containm
 ent properties of quasi-regular representations of stabilizer subgroups of
  boundary actions"\n\nA continuous action of a group G on a compact space 
 X is said to be a boundary action if the weak*-closure of the orbit of eve
 ry Borel probability on X under G-action contains all point measures on X.
  Given a boundary action of a discrete countable group\, we prove that at 
 any continuity point of the stabilizer map\, the quasi-regular representat
 ion of the stabilizer subgroup is weakly equivalent to every representatio
 n that it weakly contains. We also completely characterize when these quas
 i-regular representations weakly contain the GNS representation of a chara
 cter on the group.\nThis is joint work with Eduardo Scarparo.\n
LOCATION:https://researchseminars.org/talk/GroupTheoryENS/4/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Jingyin Huang (Ohio State University)\, Jérémie Chalopin (Aix-Ma
 rseille Université)\, Daniel Wise (McGill University)
DTSTART:20210223T143000Z
DTEND:20210223T173000Z
DTSTAMP:20260422T225826Z
UID:GroupTheoryENS/5
DESCRIPTION:Title: <a href="https://researchseminars.org/talk/GroupTheoryE
 NS/5/">A group theory and CAT(0) cubical afternoon</a>\nby Jingyin Huang (
 Ohio State University)\, Jérémie Chalopin (Aix-Marseille Université)\, 
 Daniel Wise (McGill University) as part of ENS group theory seminar\n\n\nA
 bstract\nJingyin Huang  "Morse quasiflats"\n\nWe are motivated by looking 
 for traces of hyperbolicity in a space or\ngroup which is not Gromov-hyper
 bolic. One previous approach in this\ndirection is the notion of Morse qua
 sigeodesics\, which describes\n``negatively-curved'' directions in the spa
 ces\; another previous\napproach is ``higher rank hyperbolicity'' with one
  example being that\nthough triangles in products of two hyperbolic planes
  are not thin\,\ntetrahedrons made of minimal surfaces are ``thin''. We in
 troduce the\nnotion of Morse quasiflats\, which unifies these two seemingl
 y\ndifferent approaches and applies to a wider range of objects. In the\nt
 alk\, we will provide motivations and examples for Morse quasiflats\,\nas 
 well as a number of equivalent definitions and quasi-isometric\ninvariance
  (under mild assumptions). We will also show that Morse\nquasiflats are as
 ymptotically conical\, and comment on potential\napplications. Based on jo
 int work with B. Kleiner and S. Stadler.\n\nJérémie Chalopin (TBA)\n\nDa
 niel Wise (TBA)\n
LOCATION:https://researchseminars.org/talk/GroupTheoryENS/5/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Hanna Oppelmayer (TU Graz)\, Georgii Veprev (St-Petersburg)\, Paul
 -Henry Leemann (University of  Neuchâtel)
DTSTART:20210330T120000Z
DTEND:20210330T150000Z
DTSTAMP:20260422T225826Z
UID:GroupTheoryENS/6
DESCRIPTION:Title: <a href="https://researchseminars.org/talk/GroupTheoryE
 NS/6/">An afternoon on random walks and amenable groups</a>\nby Hanna Oppe
 lmayer (TU Graz)\, Georgii Veprev (St-Petersburg)\, Paul-Henry Leemann (Un
 iversity of  Neuchâtel) as part of ENS group theory seminar\n\n\nAbstract
 \nHanna Oppelmayer\,   "Random walks on dense subgroups of  totally discon
 nected locally compact  groups"\n\nThere is a class of random walks on som
 e countable discrete groups that capture the asymptotic behaviour of certa
 in random walks\non totally disconnected locally compact second countable 
 (t.d.l.c.) groups which are completions of the discrete group. We will see
  that\nthe Poisson boundary of the t.d.l.c. group is always a factor of th
 e Poisson boundary of the discrete group\, when equipped with these\nrando
 m walks. All this is done by means of a so-called Hecke subgroup.\nIn part
 icular\, if the two Poisson boundaries are isomorphic then this Hecke subg
 roup is forced to be amenable. The reverse direction holds\nwhenever there
  is a uniquely stationary compact model for the Poisson boundary of the di
 screte group. Furthermore\, we will deduce some\napplications to concrete 
 examples\, like the lamplighter group over Z and solvable Baumslag-Solitar
  groups and show that they are prime\,\ni.e. there are random walks such t
 hat the Poisson boundary and the one-point-space are the only boundaries.\
 nThis is a joint work with Michael Björklund (Chalmers\, Sweden) and\nYai
 r Hartman (Ben Gurion University\, Israel).\n\n\nGeorgi Veprev\,  "Non-exi
 stence of a universal zero entropy system for non-periodic amenable group 
 actions"\n\nLet G be a discrete amenable group. We study interrelations be
 tween topological and measure-theoretic actions of G. For a given continuo
 us representation of G on a compact metric space X we consider the set of 
 all ergodic invariant measures on X. For any such measure we associate the
  corresponding measure-theoretic dynamical system. The general wild questi
 on is what the family M of these systems could be up to measure-theoretic 
 isomorphisms.\nThe topological system for which M coincides with a given c
 lass S of ergodic actions is called universal. B.Weiss's question regards 
 the existence of a universal system for the class of all zero-entropy acti
 ons. For the case of Z\, the negative answer was given by J. Serafin.\nOur
  main result establishes the non-existence of a universal zero-entropy sys
 tem for any non-periodic amenable group. The condition of non-periodicity 
 is crucial in our arguments so the question is still open for general tors
 ion amenable groups.\nOur proof bases on the slow entropy type invariant c
 alled scaling entropy introduced by A. Vershik. This invariant characteriz
 es the intermediate growth of the entropy in a sense on the verge of topol
 ogical and measure-preserving dynamics. I will present a brief survey of s
 caling entropy and show how this invariant applies to the non-existence th
 eorem.\n\n\nPaul-Henry Leemann\,  "De Bruijn graphs\, spider web graphs an
 d Lamplighter groups"\n\nDe Bruijn graphs represent word overlaps in symbo
 lic dynamical systems. They naturally occur in dynamical systems and combi
 natorics\, as well as in computer science and bioinformatics. We will show
  that de Bruijn graphs converge to a Cayley graph of the Lamplighter group
  and and will also compute their spetra. We will then discuss some general
 izations of them as for examples Spider web graphs or Rauzy graphs.\nBased
  on a joint work with R. Grigorchuk and T. Nagnibeda.\n
LOCATION:https://researchseminars.org/talk/GroupTheoryENS/6/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Panos Papazoglu (Oxford)\, Urs Lang (ETH Zurich)\, Karim Adiprasit
 o (Hebrew University & University of Copenhagen)
DTSTART:20210427T130000Z
DTEND:20210427T160000Z
DTSTAMP:20260422T225826Z
UID:GroupTheoryENS/7
DESCRIPTION:Title: <a href="https://researchseminars.org/talk/GroupTheoryE
 NS/7/">An afternoon on asymptotic dimension</a>\nby Panos Papazoglu (Oxfor
 d)\, Urs Lang (ETH Zurich)\, Karim Adiprasito (Hebrew University & Univers
 ity of Copenhagen) as part of ENS group theory seminar\n\n\nAbstract\n15.0
 0 - 15.45   Panos Papazoglu (Oxford)\n\n16.00 - 16.45   Urs Lang (ETH Zuri
 ch)\n\n17.15 - 18.00   Karim Adiprasito (Hebrew University & University of
  Copenhagen)\n\n\nPanos Papazoglu\, "Asymptotic dimension of planes" (join
 t with K. Fujiwara)\n\nIt is easy to see that there are Riemannian manifol
 ds homeomorphic to $\\mathbb R ^3$\nwith infinite asymptotic dimension. In
  contrast to this we showed with K. Fujiwara that\nthe asymptotic dimensio
 n of Riemannian planes (and planar graphs) is bounded by 3. This was\nimpr
 oved to 2 by Jorgensen-Lang and Bonamy-Bousquet-Esperet-Groenland-Pirot-Sc
 ott.\n\n\n\nUrs Lang\,  "Assouad-Nagata dimension and Lipschitz extensions
  "\n\nIt follows from a recent result of Fujiwara-Papasoglu and a Hurewicz
 -type theorem due to Brodskiy-Dydak-Levin-Mitra that every planar geodesic
  metric space has\n\n(Assouad-)Nagata dimension at most two and hence asym
 ptotic dimension at most two. This can be used further to prove that every
  three-dimensional Hadamard manifold \n\nhas Nagata dimension three and is
  an absolute Lipschitz retract (joint work with Martina Jørgensen). The r
 ole of the Nagata dimension in Lipschitz extension problems\nwill be discu
 ssed further.\n\n\nKarim Adiprasito\, "l^2 cohomology and stable Lefschetz
  theory"\n
LOCATION:https://researchseminars.org/talk/GroupTheoryENS/7/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Giulio Tiozzo (Toronto)\, Sébastien Gouëzel (Rennes)\, Andrei Al
 peev (St-Petersburg)
DTSTART:20210525T133000Z
DTEND:20210525T163000Z
DTSTAMP:20260422T225826Z
UID:GroupTheoryENS/8
DESCRIPTION:Title: <a href="https://researchseminars.org/talk/GroupTheoryE
 NS/8/">An afternoon on random walks and groups</a>\nby Giulio Tiozzo (Toro
 nto)\, Sébastien Gouëzel (Rennes)\, Andrei Alpeev (St-Petersburg) as par
 t of ENS group theory seminar\n\n\nAbstract\n15.30 - 16.15   Giulio Tiozzo
  (Toronto)\n\n16.30 - 17.15  Sébastien Gouëzel (Rennes)\n\n17.45 - 18.30
   Andrei Alpeev (St-Petersburg)\n\n\nGiulio Tiozzo\,  "The fundamental ine
 quality for cocompact Fuchsian groups".\n\nA recurring question in the the
 ory of random walks on hyperbolic spaces asks whether the hitting (harmoni
 c) measures can coincide with measures of geometric origin\, such as the L
 ebesgue measure. This is also related to the inequality between entropy an
 d drift.\nFor finitely-supported random walks on cocompact Fuchsian groups
  with symmetric fundamental domain\, we prove that the hitting measure is 
 singular with respect to Lebesgue measure\; moreover\, its Hausdorff dimen
 sion is strictly less than 1.\nAlong the way\, we prove a purely geometric
  inequality for geodesic lengths\, strongly reminiscent of the Anderson-Ca
 nary-Culler-Shalen inequality for free Kleinian groups.\nJoint with P. Kos
 enko.\n\n\nSébastien Gouëzel\, "Exponential estimates for random walks w
 ithout moment conditions on\nhyperbolic spaces"\n\nConsider a random walk 
 on a nonelementary hyperbolic space (proper or  not\, but one may just thi
 nk of a free group for simplicity). It is known\nthat the walk is convergi
 ng almost surely towards a point at a boundary\,  and that the rate of esc
 ape is positive. We will discuss quantitative\nversions of these statement
 s: when can one show that these facts hold with an exponentially small pro
 bability for exceptions? While there are\nseveral such results in the lite
 rature\, the originality of our approach is that it does not require any m
 oment condition on the random walk. We\nwill discuss the main technical ne
 w idea in the case of the free group.\n\nAndrei Alpeev\,  "Examples of dif
 ferent boundary behaviour of left and right random walks on groups".\n
LOCATION:https://researchseminars.org/talk/GroupTheoryENS/8/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Chris Hruska (University of Wisconsin)\, Anthony Genevois (Montpel
 lier)\, Romain Tessera (Jussieu)
DTSTART:20210622T133000Z
DTEND:20210622T163000Z
DTSTAMP:20260422T225826Z
UID:GroupTheoryENS/9
DESCRIPTION:Title: <a href="https://researchseminars.org/talk/GroupTheoryE
 NS/9/">An afternoon on quasi-isometries of groups</a>\nby Chris Hruska (Un
 iversity of Wisconsin)\, Anthony Genevois (Montpellier)\, Romain Tessera (
 Jussieu) as part of ENS group theory seminar\n\n\nAbstract\n15.30 - 16.15 
  Chris Hruska (University of Wisconsin)\n\n16.30 - 17.15  Anthony Genevois
  (Montpellier)\n\n17.45 - 18.30  Romain Tessera (Jussieu)\n\nChris Hruska\
 ,  "Canonical splittings of relatively hyperbolic groups"\n\nA JSJ decompo
 sition is a graph of groups decomposition that allows one to classify all 
 splittings of a group over certain subgroups.  I will discuss a JSJ decomp
 osition for relatively hyperbolic groupssplitting over elementary subgroup
 s that depends only on the topology of its boundary.  This decomposition c
 ould potentially be of use forunderstanding groups that have homeomorphic 
 boundaries\, but are not necessarily quasi-isometric. (Joint work with Mat
 t Haulmark.)\n\nAnthony Genevois  "Asymptotic geometry of lamplighters ove
 r one-ended groups".\n\nAfter a general introduction to lamplighter groups
  and their asymptotic geometry\, I will describe a complete quasi-isometri
 c classification of lamplighters over one-ended finitely presented groups.
  The proof will be briefly overviewed\, and the rest of the talk will be d
 edicated to the central tool of the argument: an embedding theorem proved 
 thanks to (quasi-)median geometry.\n\nRomain Tessera "Asymptotic geometry 
 of lamplighters over one-ended groups II".\n\nThis second talk will be ded
 icated to the asymmetry between amenable and non-amenable groups in the qu
 asi-isometric classification previously described. In particular\, I will 
 explain why lamplighters over non-amenable groups are more often quasi-iso
 metric than lamplighters over amenable groups. Also\, I will show how the 
 distance from a quasi-isometry between amenable groups to a bijection can 
 be quantified\, introducing quasi-k-to-one quasi-isometries for an arbitra
 ry real k>0\, and explain how this notion is fundamental in the understand
 ing of the asymptotic geometry of lamplighters over amenable groups.\n
LOCATION:https://researchseminars.org/talk/GroupTheoryENS/9/
END:VEVENT
BEGIN:VEVENT
SUMMARY:François Le Maître (Université Paris Diderot -Paris VII)\, Roma
 in Tessera (Université Paris Diderot -Paris VII)\, Pierre Fima (Universit
 é Paris Diderot -Paris VII)
DTSTART:20211011T120000Z
DTEND:20211011T150000Z
DTSTAMP:20260422T225826Z
UID:GroupTheoryENS/10
DESCRIPTION:Title: <a href="https://researchseminars.org/talk/GroupTheoryE
 NS/10/">Group theory afternoon</a>\nby François Le Maître (Université P
 aris Diderot -Paris VII)\, Romain Tessera (Université Paris Diderot -Pari
 s VII)\, Pierre Fima (Université Paris Diderot -Paris VII) as part of ENS
  group theory seminar\n\nLecture held in ENS\, 45 rue d'Ulm\, room "salle 
 W"\, roof of the DMA.\n\nAbstract\n14.00 - 14.45    François Le Maître (
 Université Paris Diderot -Paris VII)\n\n15.00- 15.45  Romain Tessera (Uni
 versité Paris Diderot -Paris VII)\n\n16.00- 16.45  Pierre Fima (Universit
 é Paris Diderot -Paris VII)\n\n\nFrançois Le Maître  "Reconstruction fo
 r Boolean measure-preserving actions of full groups and applications"\n\nG
 iven a two measure-preserving ergodic action of  countable groups on a sta
 ndard probability space\, Dye's reconstruction theorem asserts that any is
 omorphism between the associated full groups must come from an isomorphism
  of the space which sends the first partition of the space into orbits to 
 the second. It is thus natural to ask what happens more generally for homo
 morphisms between full groups. I will present a joint work with Alessandro
  Carderi and Alice Giraud where we show that any such homomorphism  comes 
 from a measure-preserving action of the equivalence relation or of one of 
 its symmetric powers. Such a result is very similar in spirit to Matte Bon
 's striking classification of actions by homeomorphisms of topological ful
 l groups\, but we will see that the proof is much simpler modulo the Thoma
 s-Tucker-Drob classification of invariant random subgroups of the dyadic s
 ymmetric group. As an application\, we characterize Kazhdan's property (T)
  of a measure-preserving equivalence relation in terms of its full group: 
 the equivalence relation has (T) if and only if all non-free ergodic Boole
 an actions of its full group are strongly ergodic.\n\n\nRomain Tessera "Co
 arse geometry meets measured group theory" .\n\n We will present a new ind
 uction technique based on ideas of Gromov and Shalom. Given two finitely g
 enerated groups H and G and a Lipschitz injective map from H to G\, we con
 struct a topological coupling space between them. If H is amenable\, then 
 this enables us to view H as a ``measured subgroup" of G. Using this forma
 lism\, we manage to prove that the Folner function of G grows faster than 
 the Folner function of H.\n\nAn application of this result is the followin
 g (new) theorem: an amenable group coarsely embeds into a hyperbolic group
  if and only it is virtually nilpotent.\n\n\nPierre Fima\, "Highly transit
 ive groups among groups acting on trees".\n\nAfter an introduction to the 
 topic of highly transitive groups\, I will present a joint work with F. Le
  Maître\, S. Moon and Y. Stalder in which we characterize groups acting o
 n trees which are highly transitive.\n
LOCATION:https://researchseminars.org/talk/GroupTheoryENS/10/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Piotr Przytycki (McGill\; on sabbatical leave at Paris-Saclay)\, S
 ami Douba (McGill\; visiting Paris-Saclay)\, Jean Lecureux (Paris-Saclay)
DTSTART:20211115T130000Z
DTEND:20211115T160000Z
DTSTAMP:20260422T225826Z
UID:GroupTheoryENS/11
DESCRIPTION:Title: <a href="https://researchseminars.org/talk/GroupTheoryE
 NS/11/">An afternoon of CAT(0) spaces and group theory</a>\nby Piotr Przyt
 ycki (McGill\; on sabbatical leave at Paris-Saclay)\, Sami Douba (McGill\;
  visiting Paris-Saclay)\, Jean Lecureux (Paris-Saclay) as part of ENS grou
 p theory seminar\n\n\nAbstract\n14.00 - 14.45 Piotr Przytycki (McGill Univ
 ersity\; on sabbatical leave at Paris-Saclay)\n\n15.00- 15.45 Sami Douba (
 McGill University\; visiting Paris-Saclay)\n\n16.00- 16.45 Jean Lecureux (
 Paris-Saclay) \n\n\nPiotr Przytycki\, "Groups acting almost freely on 2-di
 mensional CAT(0) complexes satisfy the Tits Alternative"\n\nLet X be a 2-d
 imensional complex with piecewise smooth Riemannian metric\, finitely many
  isometry types of cells\, that is CAT(0). Let G be a group acting on X wi
 th a bound on cell stabilisers. We will sketch the proof of the Tits Alter
 native saying that G is virtually cyclic\, virtually Z^2 or contains a non
 abelian free group. This generalises our earlier work for X a 2-dimensiona
 l systolic complex or a 2-dimensional Euclidean building. This is joint wo
 rk with Damian Osajda.\n\n\nSami Douba "Proper CAT(0) actions of unipotent
 -free linear groups".\n\nButton observed that finitely generated matrix gr
 oups containing no nontrivial unipotent matrices behave much like groups a
 dmitting proper actions by semisimple isometries on complete CAT(0) spaces
 . It turns out that any finitely generated matrix group possesses an actio
 n on such a space whose restrictions to unipotent-free subgroups are in so
 me sense tame. We discuss this phenomenon and some of its implications for
  the representation theory of certain 3-manifold groups.\n\n\nJean Lecureu
 x\,  "Rigidity properties of Ã_2 lattices".\n\nBuildings of type Ã_2 are
  commonly associated to groups such as G=SL_3(k)\, where k is a non-archim
 edean local field. Lattices in such a group G have strong rigidity propert
 ies (for example\, they satisfy Margulis' superrgidity). But there are als
 o buildings for which the automorphism group is smaller\, and much less un
 derstood - but in some cases still cocompact. In this talk I will explain 
 how these other "exotic" lattices are still very rigid\, and raise some op
 en questions.\n
LOCATION:https://researchseminars.org/talk/GroupTheoryENS/11/
END:VEVENT
BEGIN:VEVENT
SUMMARY:MurphyKate Montee (Carleton College)\, Tsung-Hsuan Tsai (IRMA\, CN
 RS\, Université de Strasbourg)\, Damian Orlef (IMPAN\, Warsaw)
DTSTART:20211214T140000Z
DTEND:20211214T170000Z
DTSTAMP:20260422T225826Z
UID:GroupTheoryENS/12
DESCRIPTION:Title: <a href="https://researchseminars.org/talk/GroupTheoryE
 NS/12/">An afternoon on random groups</a>\nby MurphyKate Montee (Carleton 
 College)\, Tsung-Hsuan Tsai (IRMA\, CNRS\, Université de Strasbourg)\, Da
 mian Orlef (IMPAN\, Warsaw) as part of ENS group theory seminar\n\n\nAbstr
 act\n15.00 - 15.45  MurphyKate Montee (Carleton College)\n\n16.00 -16.45  
 Tsung-Hsuan Tsai (IRMA\, CNRS\, Université de Strasbourg)\n\n17.15 - 18.0
 0  Damian Orlef (IMPAN)\n\nMurphyKate Montee\, "Cubulating Random Groups a
 t Densities d<3/14"\n\nRandom groups are one way to study "typical" behavi
 or of groups. In the Gromov density model\, we often find a threshold dens
 ity above which a property is satisfied with probability 1\, and below whi
 ch it is satisfied with probability 0. Two properties of random groups tha
 t have studied are cubulation and Property (T). In this setting these are 
 mutually exclusive\, but the threshold densities are not known. In this ta
 lk I'll present a method to demonstrate cubulation on groups with density 
 less than 3/14\, and discuss how this might be extended to demonstrate cub
 ulation for densities up to 1/4. In particular\, I will describe a constru
 ction of walls in the Cayley complex X which give rise to a non-trivial ac
 tion by isometries on a CAT(0) cube complex. This extends results of Olliv
 ier-Wise and Mackay-Przytycki at\ndensities less than 1/5 and 5/24\, respe
 ctively.\n\n\nTsung-Hsuan Tsai\, "Freiheitssatz for the density model of r
 andom groups"\n\nMagnus' Freiheitssatz states that if a group is defined b
 y a presentation with m generators and a single cyclically reduced relator
 \, and this relator contains the last generating letter\, then the first m
 -1 letters freely generate a free subgroup. We study an analogue of this t
 heorem in the Gromov density model of random groups\, showing a phase tran
 sition phenomenon at density d_r = min{1/2\, 1-log_{2m-1}(2r-1)} with 0<r<
 m: we prove that for a random group with m generators at density d\, if d<
 d_r then the first r letters freely generate a free subgroup\; whereas if 
 d>d_r then the first r letters generate the whole group.\n\n\nDamian Orlef
 \, "Non-orderability of random triangular groups by using random 3CNF form
 ulas"\n\nA random group in the triangular binomial model $\\Gamma(n\,p)$ i
 s given by the presentation $\\langle S|R \\rangle$\, where $S$ is a set o
 f $n$ generators and $R$ is a random set of cyclically reduced relators of
  length 3 over $S$\, with each relator included in $R$ independently with 
 probability $p$. When $n\\rightarrow\\infty$\, the asymptotic properties o
 f groups in $\\Gamma(n\,p)$ vary widely with the choice of $p=p(n)$. By An
 toniuk-Łuczak-Świątkowski and Żuk\, there exist constants $C\, C'$\, s
 uch that a random triangular group is asymptotically almost surely (a.a.s.
 ) free if $p<Cn^{-2}$ and a.a.s. infinite\, hyperbolic\, but not free\, if
  $p\\in (C'n^{-2}\, n^{-3/2-\\varepsilon})$. We generalize the second stat
 ement by finding a constant $c$ such that if $p\\in(cn^{-2}\, n^{-3/2-\\va
 repsilon})$\, then a random triangular group is a.a.s. not left-orderable.
  We prove this by linking left-orderability of $\\Gamma \\in \\Gamma(n\,p)
 $ to the satisfiability of the random propositional formula\, constructed 
 from the presentation of $\\Gamma$. The left-orderability of quotients wil
 l\nbe also discussed.\n
LOCATION:https://researchseminars.org/talk/GroupTheoryENS/12/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Friedrich Martin Schneider (Freiberg)\, Eduardo Scarparo (Federal 
 University of Santa Catarina)\, Gidi Amir (Bar Ilan)
DTSTART:20220111T140000Z
DTEND:20220111T170000Z
DTSTAMP:20260422T225826Z
UID:GroupTheoryENS/13
DESCRIPTION:Title: <a href="https://researchseminars.org/talk/GroupTheoryE
 NS/13/">An afternoon on amenable groups</a>\nby Friedrich Martin Schneider
  (Freiberg)\, Eduardo Scarparo (Federal University of Santa Catarina)\, Gi
 di Amir (Bar Ilan) as part of ENS group theory seminar\n\n\nAbstract\n15.0
 0 - 15.45 	Friedrich Martin Schneider (Freiberg)\n\n16.00 -16.45      Edua
 rdo Scarparo (Federal University of Santa Catarina)\n\n17.15 - 18.00    Gi
 di Amir (Bar Ilan) \n\n\nFriedrich Martin Schneider\,  "Concentration of i
 nvariant means"\n\nIn the context of large (non-locally compact) topologic
 al groups\, one frequently witnesses an extreme form  of amenability: extr
 eme amenability. A topological group G is called extremely amenable if eve
 rycontinuous action of G on a non-void compact Hausdorff space admits afix
 ed point. Most of the currently known manifestations of this phenomenon ha
 ve been exhibited using either structural Ramsey theory\, or concentration
  of measure. The talk will be focused on the latter method. Among other th
 ings\, I will discuss a new concentration result for convolution products 
 of invariant means\, based on a suitable adaptation of Azuma's inequality.
  Furthermore\, I will show how this result can beused to prove extreme ame
 nability of certain topological groups arising from von Neumann's continuo
 us geometries.\n\n\nEduardo Scarparo "Amenability and unitary representati
 ons of groups of dynamical origin.\n\n In the first half\, we report on jo
 int work with Mehrdad Kalantar in which we completely characterize C*-simp
 licity of quasi-regular representations associated to stabilizers of bound
 ary actions in terms of amenability of the isotropy groups of the groupoid
  of germs of the action. For quasi-regular representations associated to "
 open" stabilizers\, a complete characterization of C*-simplicity is still 
 missing\, and we illustrate this fact with an ad hoc proof that\, for Thom
 pson's group F < T\, the quasi-regular representation of T associated to [
 F\,F] properly weakly contains the one associated to F (a year ago Kalanta
 r spoke at this seminar  and I will emphasize the new results and examples
  obtained since then).In the second half\, we show that the topological fu
 ll group of a minimal action on the Cantor set is C*-simple if and only if
  the alternating full group is non-amenable. We use this to conclude that\
 , e.g.\, for free actions of groups of subexponential growth\, non-amenabi
 lity of the topological full group is equivalent to C*-simplicity\, but in
  general this equivalence is an open problem.\n\n\nGidi Amir  "Amenability
  of quadratic activity automata groups".\n\n  Automata groups are a family
  of groups acting on rooted trees that have a simple definition yet exhibi
 t a very rich behavior. Automaton groups include many interesting examples
  such as Grigorchuk groups\, the Basilica group\, Hanoi tower groups and l
 amplighter groups. \n\nThe activity of an automaton group\, introduced by 
 Sidki\, can be viewed as  a measure of complexity that can  grow either po
 lynomially (with some degree)  or exponentially. Sidki proved that polynom
 ial activity automata groups do not contain free subgroups\, which prompte
 d him to ask “Are all polynomial activity automata groups amenable?”\n
 \nThis was answered positively  for degree 0 (“bounded”) by Bartholdi-
 Kaimanovich-Nekrashevych and for degree 1 (“linear”) by Amir-Angel-Vir
 ag.\n\nJuschenko\, Nekrashevych and de la Salle gave a general approach al
 lowing to deduce the amenability of  groups from recurrence of the orbital
  Schreier graphs of group actions satisfying  some conditions. This allowe
 d\, among other things\, to reprove the amenability of automata groups of 
 degree 0 and 1\, and to prove the conditional result that if the "natural"
  action of a quadratic activity (d=2) automata group is recurrent then it 
 is amenable.\n\nIn recent work with Omer Angel and Balint Virag\, we prove
  that the natural Schreier graphs of the quadratic activity mother groups\
 , a special family into which all quadratic activity automata groups can b
 e embedded\, is recurrent. This allows us to conclude the amenability of a
 ll quadratic activity automata groups.The proof relies on bounding the ele
 ctrical resistance between vertices in the Schreier graphs\, which in turn
  relies on a "combinatorial" analysis of the graph structure together with
   new Nash-Williams type lower bound on resistances.\n\nAfter surveying so
 me background on automata groups\, mother groups and  electrical resistanc
 e\, and some previous amenability results on automata groups\, we will foc
 us on the new analysis giving the resistance lower bounds. No previous kno
 wledge on random walks\, automata groups or electrical resistance will be 
 assumed. This talk is based on joint work with O. Angel and B. Virag.\n
LOCATION:https://researchseminars.org/talk/GroupTheoryENS/13/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Tsachik Gelander (Weizmann Institute)\, Matthieu Joseph (ENS Lyon)
 \, Yair Hartman (Ben Gurion University)
DTSTART:20220208T140000Z
DTEND:20220208T170000Z
DTSTAMP:20260422T225826Z
UID:GroupTheoryENS/14
DESCRIPTION:Title: <a href="https://researchseminars.org/talk/GroupTheoryE
 NS/14/">An afternoon on invariant and stationary random subgroups</a>\nby 
 Tsachik Gelander (Weizmann Institute)\, Matthieu Joseph (ENS Lyon)\, Yair 
 Hartman (Ben Gurion University) as part of ENS group theory seminar\n\n\nA
 bstract\n15.00 - 15.45    Tsachik Gelander (Weizmann Institute)\, "Station
 ary\nrandom discrete subgroups of semisimple Lie groups"\n\n16.00 - 16.45 
    Matthieu Joseph (ENS Lyon)\, "Allosteric actions of\nsurface groups"\n\
 n17.15 - 18.00     Yair Hartman (Ben Gurion University)\,\n"Intersectional
  Invariant Random Subgroups"\n\nPlease see details for talks at the follow
 ing link:\nhttps://sites.google.com/site/annaerschler/grseminar\n
LOCATION:https://researchseminars.org/talk/GroupTheoryENS/14/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Marcin Sabok (McGill University)\, Juan Paucar (Jussieu)\, Josh Fr
 isch (l'ENS\, Paris)
DTSTART:20200315T130000Z
DTEND:20200315T160000Z
DTSTAMP:20260422T225826Z
UID:GroupTheoryENS/15
DESCRIPTION:Title: <a href="https://researchseminars.org/talk/GroupTheoryE
 NS/15/">A group theory afternoon</a>\nby Marcin Sabok (McGill University)\
 , Juan Paucar (Jussieu)\, Josh Frisch (l'ENS\, Paris) as part of ENS group
  theory seminar\n\nAbstract: TBA\n
LOCATION:https://researchseminars.org/talk/GroupTheoryENS/15/
END:VEVENT
BEGIN:VEVENT
SUMMARY:Marcin Sabok (McGill University)\, Juan Paucar (Jussieu)\, Josh Fr
 isch (l'ENS\, Paris)
DTSTART:20220315T130000Z
DTEND:20220315T160000Z
DTSTAMP:20260422T225826Z
UID:GroupTheoryENS/16
DESCRIPTION:Title: <a href="https://researchseminars.org/talk/GroupTheoryE
 NS/16/">A group theory afternoon</a>\nby Marcin Sabok (McGill University)\
 , Juan Paucar (Jussieu)\, Josh Frisch (l'ENS\, Paris) as part of ENS group
  theory seminar\n\nLecture held in Room W\, ENS\, Paris.\n\nAbstract\n14.0
 0-14.45  Marcin Sabok (McGill University)\n\n15.00 -15.45     Juan Paucar 
 (Jussieu)\n\n16.00 - 16.45    Josh Frisch (l'ENS\, Paris)\n\n\nMarcin Sabo
 k\, "Hyperfiniteness at hyperbolic boundaries".  I will discuss recent res
 ults establishing hyperfiniteness of the equivalence relations induced by 
 actions on the Gromov boundaries of various hyperbolic spaces. This includ
 es boundary actions of hyperbolic groups (joint work with T. Marquis) and 
 actions of the mapping class group on the boundaries of the arc graph and 
 the curve graph (joint work with P. Przytycki).\n\n\n\n\n\nJuan Paucar\, "
 Coarse embeddings between locally compact groups and quantitative measured
  equivalence". I will discuss about quantitative versions of Measure Equiv
 alence for locally compact compactly generated groups\, a notion introduce
 d by Tessera\, Le Maître\, Delabie and Koivisto on the finitely generated
  case. Moreover\, they introduced as well quantitative asymmetric versions
  of it\, called $L^p$-measured subgroups\, and in particular they proved t
 hat coarse embeddings between amenable groups imply the existence of a $L^
 \\infty$-measured coupling. In this talk\, I will prove the same statement
  on the locally compact case\, which will gives us an obstruction to coars
 e embeddings for locally compact compactly generated groups.\n\nJosh Frisc
 h\,   "Characteristic Measures and Minimal Subdynamics"\n
LOCATION:https://researchseminars.org/talk/GroupTheoryENS/16/
END:VEVENT
END:VCALENDAR