Geometry and Topology Seminar 2019-2020

The Geometry and Topology seminar meets in room 901 of Van Vleck Hall on Fridays from 1:20pm - 2:10pm.
For more information, contact Richard Kent.

Fall 2014

date	speaker	title	host(s)
August 29	Yuanqi Wang	Liouville theorem for complex Monge-Ampere equations with conic singularities.	Wang
September 5
September 12	Chris Davis (UW-Eau Claire)	L2 signatures and an example of Cochran-Harvey-Leidy	Maxim
September 19	Ben Knudsen (Northwestern)	Rational homology of configuration spaces via factorization homology	Ellenberg
September 26
October 3	Kevin Whyte (UIC)	Quasi-isometric embeddings of symmetric spaces	Dymarz
October 10	Alden Walker (UChicago)	Roots, Schottky Semigroups, and a proof of Bandt's Conjecture	Dymarz
October 17
October 24
October 31	Jing Tao (Oklahoma)	Growth Tight Actions	Kent
November 1	Young Geometric Group Theory in the Midwest Workshop
November 7	Thomas Barthelmé (Penn State)	Counting orbits of Anosov flows in free homotopy classes	Kent
November 14	Alexandra Kjuchukova (University of Pennsylvania)	On the classification of irregular branched covers of four-manifolds	Maxim
November 21
Thanksgiving Recess
December 4, Thursday at 4pm in VV 901	Oyku Yurttas (Georgia Tech)	Dynnikov and train track transition matrices of pseudo-Anosov braids	Thiffeault
December 5	No seminar.
December 12	No seminar.

Fall Abstracts

Yuanqi Wang

Liouville theorem for complex Monge-Ampere equations with conic singularities.

Following Calabi, Pogorelov, Evans-Krylov-Safanov, and Trudinger's pioneer work on interior regularities and liouville theorems for Monge-Ampere equations, we prove the Liouville theorem for conic Kähler-Ricci flat metrics. We also discuss various applications of this Liouville theorem to conic Kähler geometry.

Chris Davis (UW-Eau Claire)

L² signatures and an example of Cochran-Harvey-Leidy

Ben Knudsen (Northwestern)

Rational homology of configuration spaces via factorization homology

The study of configuration spaces is particularly tractable over a field of characteristic zero, and much effort has gone into producing chain complexes simple enough for explicit computations, formulas for Betti numbers, and homological stability results. I will discuss recent work identifying the homology of the configuration spaces of an arbitrary manifold M with the homology of a certain Lie algebra constructed from the compactly supported cohomology of M. The aforementioned results follow immediately from this identification, albeit with hypotheses removed; in particular, one obtains a new, elementary proof of homological stability for configuration spaces.

Kevin Whyte (UIC)

The rigidity theorems of Mostow and Margulis for lattices in semi simple Lie groups are some of the most celebrated in their field, and are motivation for much of geometric group theory. Mostow's result, which states that every isomorphism between lattices extends to an equivariant isometry between symmetric spaces, has been generalized by Kleiner and Leeb to say that any map between higher rank symmetric spaces which is quasi-isometric (a large scale version of bilipschitz) is actually a perturbation of an isometry. Margulis' superrigidity theorem, which says every homomorphism between lattices which has infinite image extends to an equivariant isometric embedding of symmetric spaces, has resisted a generalization of this sort. We will discuss one such result, which considers when quasi-isometric embeddings of symmetric spaces are near isometric embeddings. Our results show that the situation is complicated - in some cases one does have rigidity while in others there are exotic quasi-isometric embeddings. What geometric properties distinguish the two cases is only starting to be understood. This is joint work with David Fisher (Indiana).

Alden Walker (UChicago)

In 1985, Barnsley and Harrington defined a "Mandlebrot set" M for pairs of complex dilations. This is the set of complex numbers c such that the limit set generated by the pair of dilations x-> cx+1 and x-> cx-1 is connected. The set M is also the closure of the set of roots of polynomials with coefficients in {-1,0,1}. As with the usual Mandlebrot set, M has strong connections to dynamics and algebra, and it has been studied by Bousch, Bandt, Solomyak, Xu, Thurston, and Tiozzo. For a geometric group theorist, the study of M is qualitatively similar to the study of Kleinian groups acting on their limit sets or on universal circles.

Barnsley and Harrington noted the (numerically apparent) existence of infinitely many "holes" in M, which correspond to exotic components of the space of Schottky semigroups. Bandt rigorously confirmed a single hole in 2002 and conjectured that the interior of M is dense in M away from the real axis. We give the new technique of "traps" to certify an interior point of M, and we use these traps to prove Bandt's conjecture and certify the existence of infinitely many holes in M.

The only prerequisite for this talk is point-set topology. Fun pictures will be provided. This is joint work with Danny Calegari and Sarah Koch.

Jing Tao (Oklahoma)

Growth Tight Actions

Let G be a group equipped with a finite generating set S. G is called growth tight if its exponential growth rate relative to S is strictly greater than that of every quotient G/N with N infinite. This notion was first introduced by Grigorchuk and de la Harpe. Examples of groups that are growth tight include free groups relative to bases and, more generally, hyperbolic groups relative to any generating set. In this talk, I will provide some sufficient conditions for growth tightness which encompass all previous known examples.

Thomas Barthelmé (Penn State)

Counting orbits of Anosov flows in free homotopy classes

In 1972, Plante and Thurston asked the following question: If M is a manifold supporting an Anosov flow, does the number of conjugacy classes in the fundamental group grows exponentially fast with the length of the shortest orbit representative? This question can be solved by answering the following: Can one give an upper bound on the growth rate of the length of orbits inside a free homotopy class?

In this talk, I will explain how one can use the geometry and topology of Anosov flows to answer both questions in the 3-manifold case. This is joint work with Sergio Fenley.

Alexandra Kjuchukova (University of Pennsylvania)

On the classification of irregular branched covers of four-manifolds

It is a famous result of Hilden and Montesinos that every closed orientable three-manifold can be realized as an irregular three-fold cover of S^3 branched over a knot. In contrast, a smooth four-manifold which can be realized as a cover of S^4 branched over a smooth surface must have signature equal to zero. Given two simply-connected, closed, oriented four-manifolds X and Y and a surface B embedded in X with an isolated singularity, I will prove a necessary condition for the existence of an irregular dihedral branched covering map f: Y -> X with branching set B. Conversely, given a simply-connected oriented closed four-manifold X, I will outline a construction realizing as irregular dihedral covers of X infinitely many (and conjecturally all) of the manifolds Y afforded by the necessary condition.

Oyku Yurttas (Georgia Tech)

Dynnikov and train track transition matrices of pseudo-Anosov braids

In this talk we will compare a Dynnikov matrix with the train track transition matrix of a given pseudo-Anosov braid on the finitely punctured disk. Our main result is that these matrices are isospectral up to roots of unity and some zeros under particular conditions.

Spring 2015

date	speaker	title	host(s)
January 23
January 30
February 6
February 13
February 20	Jenya Sapir (UIUC)	Counting non-simple closed curves on surfaces.	Dymarz
February 27
March 6
March 13
March 20
March 27
Spring Break
April 10
April 17	Sean Li (UChicago)	Coarse differentiation of Lipschitz functions.	Dymarz
April 24
May 1
May 8

Spring Abstracts

Jenya Sapir

Counting non-simple closed curves on surfaces.

We show how to get coarse bounds on the number of (non-simple) closed geodesics on a surface, given upper bounds on both length and self-intersection number. Recent work by Mirzakhani has produced asymptotics for the growth of the number of simple closed curves, and this work has been extended to curves with at most 3 self-intersections with respect to length. However, no asymptotics, or even bounds, were previously known for other bounds on self-intersection number. Time permitting, we will discuss some applications of this result.

Sean Li

Coarse differentiation of Lipschitz functions.

Bates, Johnson, Lindenstrauss, Preiss, and Schechtman introduced a notion of large scale differentiation for Lipschitz functions between normed linear spaces. We discuss an extension of this result to the nonabelian setting of Carnot groups and use it to derive quantitative estimates for nonembeddability of such groups into certain classes of metric spaces.

Archive of past Geometry seminars

2013-2014: Geometry_and_Topology_Seminar_2013-2014

2012-2013: Geometry_and_Topology_Seminar_2012-2013

2011-2012: Geometry_and_Topology_Seminar_2011-2012

2010: Fall-2010-Geometry-Topology