# UB Geometry and Topology Seminar

### 2018-2019

Unless noted, all seminars are on Friday at 4pm, in 122 Mathematics Building. 2017-18 seminar listing can be found here.

Sep 7 Ludwik Dabrowski (SISSA): The weak Hilbert-Smith conjecture from the Borsuk-Ulam type conjecture Abstract: We show that a conjecture of Ageev follows from the Borsuk-Ulam-type conjecture of Baum, Dabrowski and Hajac. Then we explain how the Ageev conjecture implies the weak version of the Hilbert-Smith conjecture which states that no infinite compact zero-dimensional group can act freely on a manifold so that the orbit space is finite dimensional. The Hilbert-Smith conjecture originates from the already settled Hilbert's fifth problem concerning a characterization of Lie groups. Camille Horbez (CNRS/Fields Institute): Growth under automorphisms of hyperbolic groups Abstract: Let $G$ be a finitely generated group, let $S$ be a finite generating set of $G$, and let $f$ be an automorphism of $G$. A natural question is the following: what are the possible asymptotic behaviors for the length of $f^n(g)$, written as a word in the generating set $S$, as $n$ goes to infinity, and as $g$ varies in the group $G$? We investigate this question in the case where $G$ is a torsion-free Gromov hyperbolic group. Growth was completely described by Thurston when $G$ is the fundamental group of a hyperbolic surface, and can be understood from Bestvina-Handel’s work on train-tracks when $G$ is a free group. We address the case of a general torsion-free hyperbolic group. We show in particular that every element $g$ has a well-defined exponential growth rate under iteration of $f$, and that only finitely many exponential growth rates arise as $g$ varies in $G$. This is a joint work with Rémi Coulon, Arnaud Hilion and Gilbert Levitt. Kiyoshi Igusa (Brandeis): Equivariant Hatcher construction Abstract: This is a joint project with Tom Goodwillie extending earlier joint work with Goodwillie and Ohrt. The purpose of this project is to construct all exotic smooth structures on all smooth manifold bundles with a fiberwise group action in a stable range of dimensions. I will start by defining and enumerating the exotic smooth structures. By enumerate'' I mean compute the dimension of the vector space of exotic structures. To do this, we use a simplified version of Mackey functors. We give a simple construction of these exotic structures using the irreducible real representations of a finite group $G$. We call it the equivariant Hatcher construction'' since it generalizes a classical construction due to Hatcher. We use higher Reidemeister torsion with coefficients in a Mackey functor to demonstrate that our construction (with all possible inputs) spans the vector space of all stable exotic structures on a fixed $G$-bundle. Mahan Mj (Tata Institute/Fields Institute): Bowen-Margulis measures and Extremal Cocycle Growth Abstract: We establish a connection between extreme values of stable random fields arising in probability and groups $G$ acting geometrically on CAT(-1) spaces $X$. The connection is mediated by the action of the group on its limit set equipped with the Patterson-Sullivan measure. Based on motivation from extreme value theory, we introduce an invariant of the action called extremal cocycle growth and show that its non-vanishing is equivalent to finiteness of the Bowen-Margulis measure for the associated unit tangent bundle $U(X/G)$ provided $X$ is not a tree whose edges are (up to scale) integers. We also establish an analogous statement for normal subgroups of free groups. This is joint work with Parthanil Roy. Jing Tao (University of Oklahoma/Fields Institute): Big Torelli groups Abstract: A surface $S$ is of finite-type if its fundamental group is finitely generated; otherwise, it is of infinite type. The mapping class group MCG($S$) of $S$ is the group of isotopy classes of orientation-preserving homeomorphisms of $S$. This is a well-studied group when $S$ has finite type, but big mapping class groups, i.e. MCG($S$) of infinite-type surfaces, remain quite mysterious. But big mapping class groups arise naturally in various areas of mathematics and recently there has been a surge of interests in studying them. In this talk, I will discuss some recent results about the Torelli subgroup of MCG($S$). This is joint with Aramayona, Ghaswala, Kent, McLeay, and Winarski. Bulent Tosun (Alabama): Contact surgeries, symplectic fillings and Lagrangian discs Abstract: It is well known that all contact 3-manifolds can be obtained from the standard contact structure on the 3-sphere by contact surgery on a Legendrian link. Hence, an interesting and much studied question asks what properties are preserved under various types of contact surgeries. The case for the negative contact surgeries is fairly well understood. In this talk, we will discuss some new results about positive contact surgeries and in particular completely characterize when contact ($r$)-surgery is symplectically/Stein fillable for $r\in(0,1]$. This is joint work with James Conway and John Etnyre. Jacob Russell (CUNY): Convexity in Hierarchically Hyperbolic Spaces Abstract: Convexity is a fundamental notion across a variety of flavors of geometry. In the study of the course geometry of metric spaces, it is natural to study quasiconvexity i.e. convexity with respect to quasi-geodesics. We study quasiconvexity in the class of hierarchically hyperbolic spaces; a generalization of Gromov hyperbolic spaces which contains the mapping class group, right-angled Artin and Coxeter groups, and many 3-manifold groups. ​ Inspired by the rich theory of quasiconvexity in hyperbolic spaces, we show that quasiconvex subsets of hierarchcially hyperbolic spaces mimic the behavior of quasiconvex subsets in hyperbolic spaces.

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