# Geometry and Topology Seminar

Current contact: Dave Futer or Matthew Stover

The Seminar usually takes place on Wednesdays at 2:30 PM in Room 617 on the sixth floor of Wachman Hall.

• Wednesday February 20, 2019 at 14:30, Wachman 617
Coherence and lattices

Matthew Stover, Temple University

I will survey (in)coherence of lattices in semisimple Lie groups, with a view toward open problems and connections with the geometry of locally symmetric spaces. Particular focus will be placed on rank one lattices, where I will discuss connections with reflection groups, "algebraic" fibrations of lattices, and analogies with classical low-dimensional topology.

• Friday February 15, 2019 at 16:00, Haverford College, room TBA
TBA

Francesco Lin, Princeton University PATCH Seminar (joint with Bryn Mawr, Haverford, and Penn)

Abstract TBA

• Friday February 15, 2019 at 14:30, Haverford College, room TBA
TBA

Oleg Lazarev, Columbia University

PATCH Seminar (joint with Bryn Mawr, Haverford, and Penn)

Abstract TBA

• Wednesday February 13, 2019 at 14:30, Wachman 617
Simplicial complexes, configuration spaces, and ‘chromatic’ invariants

Andrew Cooper, NC State

Given a space $X$, the configuration space $F(X,n)$ is the space of possible ways to place $n$ points on $X$, so that no two occupy the same position. But what if we allow some of the points to coincide?

The natural way to encode the allowed coincidences is as a simplicial complex $S$. I will describe how the configuration space $M(S,X)$ obtained in this way gives rise to polynomial and homological invariants of $S$, how those invariants are related to the cohomology ring $H^*(X)$, and what this has to do with the topology of spaces of maps into $X$.

I will also mention some potential applications of this structure to problems arising from international relations and economics.

This is joint work with Vin de Silva, Radmila Sazdanovic, and Robert J Carroll.

• Wednesday February 6, 2019 at 14:30, Wachman 617
Virtually torsion-free CAT(0) cubical IFP groups have uniform exponential growth

Thomas Ng, Temple University

Abstract: A group is said to have uniform exponential growth if the number of elements that can be spelled with words of bounded length is bounded below by a single exponential function over all generating sets. In 1981, Gromov asked whether all groups with exponential growing group in fact have uniform exponential growth. While this was shown not to be the case in general, it has been answered affirmatively for many natural classes of groups such as hyperbolic groups, linear groups, and the mapping class groups of a surface. In 2018, Kar-Sageev show that groups acting properly on 2-dimensional CAT(0) cube complexes by loxodromic isometries either have uniform exponential growth or are virtually abelian by explicitly exhibiting free semigroups whose generators have uniformly bounded word length whenever they exist. These free semigroups witness the uniform exponential growth of the group. I will explain how certain arrangements of hyperplane orbits can be used to build loxodromic isometries generating free semigroups and then describe how to use the convex hull of their axes and the Bowditch boundary to extend Kar and Sageev's result to CAT(0) cube complexes with isolated flats. This is joint work with Radhika Gupta and Kasia Jankiewicz.

• Wednesday January 30, 2019 at 14:30, Wachman 617
Circle packings and Delaunay circle patterns for complex projective structures

Andrew Yarmola, Princeton University

Abstract: At the interface of discrete conformal geometry and the study of Riemann surfaces lies the Koebe-Andreev-Thurston theorem. Given a triangulation of a surface $S$, this theorem produces a unique hyperbolic structure on $S$ and a geometric circle packing whose dual is the given triangulation. In this talk, we explore an extension of this theorem to the space of complex projective structures - the family of maximal $CP^1$-atlases on $S$ up to Möbius equivalence. Our goal is to understand the space of all circle packings on complex projective structures with a fixed dual triangulation. As it turns out, this space is no longer a unique point and evidence suggests that it is homeomorphic to Teichmüller space via uniformization - a conjecture by Kojima, Mizushima, and Tan. In joint work with Jean-Marc Schlenker, we show that this projection is proper, giving partial support for the conjectured result. Our proof relies on geometric arguments in hyperbolic ends and allows us to work with the more general notion of Delaunay circle patterns, which may be of separate interest. I will give an introductory overview of the definitions and results and demonstrate some software used to motivate the conjecture. If time permits, I will discuss additional ongoing work with Wayne Lam.

• Wednesday January 23, 2019 at 14:30, Wachman 617
Surfaces almost transverse to circular pseudo-Anosov flows

Michael Landry, Yale University

Let $M$ be a closed hyperbolic 3-manifold which fibers over $S^1$, and let $F$ be a fibered face of the unit ball of the Thurston norm on $H^1(M;R)$. By results of Fried, there is a nice flow on $M$ naturally associated to $F$. We study surfaces which are almost transverse to $F$ and give a new characterization of the set of homology directions of $F$ using Agol’s veering triangulation of an auxiliary cusped 3-manifold.