Group Actions and Dynamics Seminar

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During the Spring 2024 semester, RTG / Group Actions and Dynamics seminar meets in room Sterling Hall 3425 on Mondays from 2:25pm - 3:15pm. To sign up for the mailing list send an email from your wisc.edu address to dynamics+join@g-groups.wisc.edu. For more information, contact Paul Apisa, Marissa Loving, Caglar Uyanik, Chenxi Wu or Andy Zimmer.


Spring 2024

date speaker title host(s)
January 22 Aaron Messerla (UIC) Quasi-isometries of relatively hyperbolic groups with an elementary hierarchy Loving and Uyanik
January 24 (1pm in VV 901) Mitul Islam (Max-Planck-Institut) Morse-ness in convex projective geometry Zimmer
January 29 Michael Zshornack (UCSB) Rational surface groups on the Hitchin component Zimmer
February 5 Sayantan Khan (Michigan) Bootstrapping dynamics in the moduli space of non-orientable surfaces Uyanik
February 12 Noelle Sawyer (Southwestern) Unique Equilibrium States for Geodesic Flows Loving, Uyanik, Work
February 19 Yulan Qing (Tennessee) Geometric Boundary of Groups Zimmer
February 26 Blandine Galiay (IHES) Divisible convex sets in flag manifolds and rigidity Zimmer
March 4 David Aulicino (Brooklyn College) TBA Apisa
March 11 Aaron Calderon (Chicago) TBA Loving and Uyanik
March 18 Josh Southerland (Indiana) TBA Fisher
April 1 Caglar Uyanik (UW) TBA local
April 8 Matt Bainbridge (Indiana) TBA Apisa
April 15 Ilya Kapovich (CUNY) TBA Uyanik
April 22 Yu-Chan Chang (Wesleyan) TBA Dymarz
April 29 Chris Leininger (Rice) TBA Kent, Loving, Uyanik

Spring Abstracts

Aaron Messerla

Sela introduced limit groups in his work on the Tarski problem, and showed that each limit group has a cyclic hierarchy. We prove that a class of relatively hyperbolic groups, equipped with a hierarchy similar to the one for limit groups, is closed under quasi-isometry. Additionally, these groups share some of the algebraic properties of limit groups. In this talk I plan to present motivation for and introduce the class of groups studied, as well as present some of the results for this class.

Mitul Islam

The (Hilbert metric) geometry of properly convex domains generalizes real hyperbolic geometry. This generalization is far from the Riemannian notion of non-positive curvature, but they have some intriguing similarities. I will explore this connection from a coarse geometry viewpoint. The focus will be on Morse geodesics ("negatively curved directions", in a coarse sense) in properly convex domains. I will show that Morse-ness can be characterized entirely using linear algebraic data (i.e. singular values of matrices that track the geodesic). Further, I will discuss how this coarse geometric notion of Morse is related to the symmetric space geometry as well as the smoothness of boundary points. This is joint work with Theodore Weisman.

Michael Zshornack

Margulis's work on lattices in higher-rank and a number of questions on the existence of surface subgroups motivate the need for understanding arithmetic properties of spaces of surface group representations. In recent work with Jacques Audibert, we outline one possible approach towards understanding such properties for the Hitchin component, one particularly nice space of representations. We utilize the underlying geometry of this space to reduce questions about its arithmetic to questions about the arithmetic of certain algebraic groups, which in turn, allows us to characterize the rational points on these components. In this talk, I'll give an overview of the geometric methods behind the proof of our result and indicate some natural questions about the nature of the resulting surface group actions that follow.

Sayantan Khan

The moduli spaces of non-orientable hyperbolic surfaces behave significantly differently from their orientable counterparts. They have infinite volume, almost all geodesic flow orbits escape off to infinity, and the growth of mapping class group orbit points and simple closed curves is believed to have non-integer exponents, unlike in the orientable setting. In this talk, we outline some of the oddities of these moduli spaces, and outline an approach for studying the dynamics on these spaces via Patterson-Sullivan theory. A key obstruction to imitating the techniques from the orientable setting is that a number of these techniques rely on the dynamics of the geodesic flow and the mapping class group action on Teichmüller spaces of orientable surfaces, which is not something we can do in the non-orientable setting, since that is what we are trying to prove. The way around these obstructions is by proving weaker versions of these dynamical statements using a dynamics free approach, which we then use to bootstrap the stronger results.

Noelle Sawyer

In this talk I will discuss some known results about the geodesics and equilibrium states of the geodesic flow in negative curvature. After, I will introduce some of the tools and techniques needed to show the uniqueness of equilibrium states in the setting of translation surfaces. If time allows, I will talk about some of our upcoming work about the Bernoulli property. This is joint work with Benjamin Call, Dave Constantine, Alena Erchenko, and Grace Work.


Yulan Qing

Gromov boundary provides a useful compactification for all infinite-diameter Gromov hyperbolic spaces. It consists of all geodesic rays starting at a given base-point and it is an essential tool in the study of the coarse geometry of hyperbolic groups. In this talk we generalize the Gromov boundary. We first construct the sublinearly Morse boundaries and show that it is a QI-invariant, metrizable topological space. We show sublinearly Morse directions are generic both in the sense of Patterson-Sullivan and in the sense of random walk.

The sublinearly Morse boundaries are subsets of all directions with desired properties. In the second half of the talk, we will truly consider the space of all directions and show that with some minimal assumptions on the space, the resulting boundary is a QI-invariant topology space in which many existing boundaries are embedded. This talk is based on a series of work with Kasra Rafi and Giulio Tiozzo.

Blandine Galiay

Divisible convex sets have been widely studied since the 1960s. They are proper domains of the projective space that admit a cocompact action of a discrete subgroup of the linear projective group. The best-known examples are symmetric spaces embedded in the projective space, but there are also many nonsymmetric examples. It is natural to seek to generalize this theory, by replacing the projective space by a flag variety G/P, where G is a real semisimple non-compact Lie group and P a parabolic of G. A question of van Limbeek and Zimmer is then: are there examples of divisible convex sets in G/P that are nonsymmetric? In a number of cases, it has been proved that there are not. In this talk, we will focus on some particular classes of flag varieties in which rigidity can indeed be observed.

David Aulicino

Aaron Calderon

Josh Southerland

Caglar Uyanik

Matt Bainbridge

Ilya Kapovich

Yu-Chan Chang

Chris Leininger

Fall 2023

date speaker title host(s)
September 11 Vaibhav Gadre (Glasgow) Teichmuller flow detects the fundamental group Apisa
September 18 Becky Eastham (UW Madison) Whitehead space: a tool to study finite regular covers of graphs local
September 25 Brandis Whitfield (Temple) Short curves of end-periodic mapping tori Loving
September 28 (Thursday 4-5pm in B139) Itamar Vigdorovich (Weizmann) Group stability, characters, and dynamics on compact groups Dymarz/Gurevich
October 2 Hanh Vo (Arizona State) Short geodesics with self-intersections Dymarz
October 9 Yandi Wu (UW Madison) Marked Length Spectrum Rigidity for Surface Amalgams local
October 16 Sanghoon Kwak (Utah) Mapping class groups of Infinite graphs — “Big Out(Fn)” Loving
October 23 (11:55-12:55 in B223) Sara Maloni (UVA) Dynamics on the SU(2,1)-character varieties of the one-holed torus Uyanik
October 30 Giulio Tiozzo (Toronto) A characterization of hyperbolic groups via contracting elements Uyanik
November 6
November 13 Hongming Nie (Stony Brook) A metric view of polynomial shift locus Wu
November 20 Sam Freedman (Brown) Periodic points of Prym eigenforms Apisa
November 27 Luke Jeffreys (UW Madison) Non-planarity of SL(2,Z)-orbits of origamis local
December 4 Emily Stark (Wesleyan) Graphically discrete groups and rigidity Uyanik
December 11 Colloquium by Mikolaj Fraczyk at 4pm

Fall Abstracts

Vaibhav Gadre

A quadratic differential on a Riemann surface is equivalent to a half-translation structure on the surface by complex charts with half-translation transitions. The SL(2,R)-action on the complex plane takes half-translations to half-translations and so descends to moduli spaces of quadratic differentials. The diagonal part of the action is the Teichmuller flow.


Apart from its intrinsic interest, the dynamics of Teichmuller flow is central to many applications in geometry, topology and dynamics. The Konstevich—Zorich cocycle which records the action of the flow on the absolute homology of the surface, plays a key role.


In this talk, I will explain how the flow detects the topology of moduli spaces. Specifically, we will show that the flow group, namely the subgroup generated by almost flow loops, has finite index in the fundamental group. As a corollary, we will prove that the minus and plus (modular) Rauzy—Veech groups have finite index in the fundamental group, answering a question by Yoccoz.


Using this, and Filip’s results on algebraic hulls and Zariski closures of modular monodromies, we prove that the Konstevich—Zurich cocycle (separately minus and plus pieces) have a simple Lyapunov spectrum, extending the work of Forni from 2002 and Avila—Viana from 2007.

Becky Eastham

The Whitehead space of a finite regular cover of the rose is a locally infinite graph whose vertices are in one-to-one correspondence with conjugacy classes of elements of the subgroup associated with the cover. Every Whitehead space is a subgraph of the quotient of [math]\displaystyle{ \mathrm{Cay}(F_n, \mathcal{C}) }[/math] by conjugacy; here $\mathcal{C}$ is the set of elements of $F_n$ conjugate into a proper free factor. Our main interest in this space is that it is connected if and only if the fundamental group of the associated cover is generated by lifts of elements of $\mathcal{C}$ to the cover. In addition, Whitehead space of the rose is an infinite-diameter, non-hyperbolic, one-ended space with an isometric action of $\mathrm{Out}(F_n)$. Thus, Whitehead space is not quasi-isometric to the free factor complex, the free splitting complex, or Outer Space.

Brandis Whitfield

Let $S$ be a boundaryless infinite-type surface with finitely many ends and consider an end-periodic homeomorphism $f$$ $of S. The end-periodicity of $f$ ensures that $M_f$, its associated mapping torus, has a compactification as a $3$-manifold with boundary; and further, if $f$ is atoroidal, then $M_f$ admits a hyperbolic metric.

As an end-periodic analogy to work of Minsky in the finite-type setting, we show that given a subsurface $Y\subset S$, the subsurface projections between the "positive" and "negative" Handel-Miller laminations provide bounds for the geodesic length of the boundary of $Y$ as it resides in $M_f$.

In this talk, we'll discuss the motivating theory for finite-type surfaces and closed fibered hyperbolic $3$-manifolds, and how these techniques may be used in the infinite-type setting.

Itamar Vigdorovich

I will discuss three seemingly unrelated topics: 1. Stability: given a pair of matrices that almost commute, can they be perturbed to matrices which do commute? Interestingly, the answer highly depends on the chosen metric on matrices. This question is a special case of group stability: is every almost-homomorphism close to an actual homomorphism? 2. Characters: these are functions on groups with special properties that generalize the classical notion in Pontryagin's theory of abelian groups, and in Frobenius's theory of finite groups. Is every character a limit of a finite-dimensional character? 3. Topological dynamics: given a group G acting by homeomorphisms on a compact space X, are the periodic measures dense is the space all invariant measures? In this talk I will present these three subjects of study and explain how there are all in fact intimately related, as least in the amenable setting. For example, stability of the lamplighter group is strongly related to the orbit closing lemma for the Bernoulli shift, and stability of the semidirect product ZxZ[1/6] is related to whether Furstenberg's x2x3 system has dense periodic measures. The talk is based on a joint work with Arie Levit.

Hanh Vo

We consider the set of closed geodesics on a hyperbolic surface. Given any non-negative integer k, we are interested in the set of primitive essential closed geodesics with at least k self-intersections. Among these, we investigate those of minimal length. In this talk, we will discuss their self-intersection numbers.

Yandi Wu

The marked length spectrum of a negatively curved metric space can be thought of as a length assignment to every closed geodesic in the space. A celebrated result by Otal says that metrics on negatively curved closed surfaces are determined completely by their marked length spectra. In my talk, I will discuss my work towards extending Otal’s result to a large class of surface amalgams, which are natural generalizations of surfaces.

Sanghoon Kwak

Surfaces and graphs are closely related; there are many parallels between the mapping class groups of finite-type surfaces and finite graphs, where the mapping class group of a finite graph is the outer automorphism group of a free group of (finite) rank. A recent surge of interest in infinite-type surfaces and their mapping class groups begs a natural question: What is the mapping class group of an “infinite” graph? In this talk, I will explain the answer given by Algom-Kfir and Bestvina and present recent work, joint with George Domat (Rice University), and Hannah Hoganson (University of Maryland), on the coarse geometry of such groups.

Sara Maloni

In this talk we will discuss join work in progress with S. Lawton and F. Palesi on the (relative) SU(2, 1)–character variety for the once-holed torus. We consider the action of the mapping class group and describe a domain of discontinuity for this action, which strictly contains the set of convex-cocompact characters. We will also discuss the connection with the recent work of S. Schlich, and the inspiration behind this project, which lies in the rich theory developed for SL(2, C)–character varieties by Bowditch, Minsky and others.

Giulio Tiozzo

The notion of contracting element has become central in geometric group theory, singling out, in an arbitrary metric space, the geodesics which behave like geodesics in a delta-hyperbolic space. In this work, joint with K. Chawla and I. Choi, we prove the following characterization of hyperbolic groups: a group is hyperbolic if and only if the D-contracting elements are generic with respect to counting in the Cayley graph.


Hongming Nie

The escaping rates of critical points for polynomials in C[z] induce a continuous and proper map on the moduli space M_d of degree d\ge 2 polynomials. This map has a monotone-light factorization via an intermediate space T_d^* studied by DeMarco and Pilgrim. Restricting on the shift locus S_d of M_d, one obtains the corresponding intermediate space ST_d^*. In this talk, I will relate generic points in S_d to the length functions on the (2d-2)-rose graph and then present an understanding of the natural projectivization of ST_d^* from a metric view. The metric is obtained from thermodynamic metrics on the space of metric graphs. This is a joint work with Yan Mary He.

Sam Freedman

We will consider the dynamics of affine automorphisms acting on highly symmetric translation surfaces called Veech surfaces. Specifically, we’ll examine the points of the surface that are periodic, i.e., have a finite orbit under the whole automorphism group. While this set is known to be finite for primitive Veech surfaces, for applications it is desirable to determine the periodic points explicitly. In this talk we will discuss our classification of periodic points in the case of minimal Prym eigenforms, certain primitive Veech surfaces in genera 2, 3, and 4.

Luke Jeffreys

Origamis (also known as square-tiled surfaces) arise naturally in a variety of settings in low-dimensional topology. They can be thought of as generalisations of the torus (the unit square with opposite sides glued) since they are surfaces obtained by gluing the opposite sides of a collection of unit squares. There is a natural action of the matrix group SL(2,Z) on origamis. In genus two (with some extra conditions) the orbits of this action were classified by Hubert-Lelièvre and McMullen. By considering a generating set of size two for SL(2,Z) and varying the number of squares used to build the origamis, we can turn these orbits into an infinite family of four-valent graphs. It is a long-standing conjecture of McMullen that these orbit graphs form a family of expander graphs. In this talk, giving indirect evidence for this conjecture, I will discuss joint work with Carlos Matheus in which we show that these orbit graphs are eventually non-planar - a requirement of any family of expander graphs.


Emily Stark

Rigidity problems in geometric group theory frequently have the following form: if two finitely generated groups share a geometric structure, do they share algebraic structure? We consider two finitely generated groups that are either quasi-isometric or act geometrically on the same proper metric space, and we ask if they are virtually isomorphic. The work of Papasoglu--Whyte demonstrates that infinite-ended groups are quasi-isometrically flexible, but our results show that if you assume a common geometric model, then there is often rigidity. To do this, we introduce the notion of a graphically discrete group, which imposes a discreteness criterion on the automorphism group of any graph the group acts on geometrically. Classic examples of graphically discrete groups include virtually nilpotent groups and fundamental groups of closed hyperbolic manifolds; free groups are non-examples. We will present new examples and demonstrate this property is not a commensurability invariant. We will present rigidity phenomena for free products of graphically discrete groups. This is joint work with Alex Margolis, Sam Shepherd, and Daniel Woodhouse.


Archive of past Dynamics seminars

2022-2023 Dynamics_Seminar_2022-2023

2021-2022 Dynamics_Seminar_2021-2022

2020-2021 Dynamics_Seminar_2020-2021