Geometry and Topology Seminar Spring 2016

The Geometry and Topology seminar meets in room TBA of Van Vleck Hall on THURSDAY from 2:25pm - 3:15pm.
For more information, contact Tullia Dymarz or Alexandra Kjuchukova.

Spring 2016

In Spring of 2016, the seminar will meet on Thursdays at 2:25PM (location TBA).

date	speaker	title	host(s)
January 21
January 28	Ryan Blair (UCSLB)	TBA	Kjuchukova
February 4
February 11
February 18
February 25
March 3
March 10
March 17
March 24	Spring Break
March 31
April 7
April 14
April 21
April 28
May 5

Fall 2015

date	speaker	title	host(s)
September 4
September 11	Hung Tran (UW Milwaukee)	Relative divergence, subgroup distortion, and geodesic divergence	T. Dymarz
September 18	Tullia Dymarz (UW Madison)	Non-rectifiable Delone sets in amenable groups	(local)
September 25	Jesse Wolfson (Uchicago)	Counting Problems and Homological Stability	M. Matchett Wood
October 2	Jose Ignacio Cogolludo Agustín (University of Zaragoza, Spain)	Topology of curve complements and combinatorial aspects	L. Maxim
October 9	Matthew Cordes (Brandeis)	Morse boundaries of geodesic metric spaces	T. Dymarz
October 16	Jacob Bernstein (Johns Hopkins University)	Hypersurfaces of low entropy	L. Wang
October 23	Yun Su (UW Madison)	Higher-order degrees of hypersurface complements.	(local)
October 30	Gao Chen (Stony Brook University)	Classification of gravitational instantons	B.Wang
November 6	Dan Cristofaro-Gardiner (Harvard)	Higher-dimensional symplectic embeddings and the Fibonacci staircase	Kjuchukova
November 13	Danny Ruberman (Brandeis)	Configurations of embedded spheres	Kjuchukova
November 20	Anton Izosimov (University of Toronto)	TBA	Mari-Beffa
Thanksgiving Recess
December 4	Quinton Westrich (UW Madison)	Harmonic Chern Forms on Polarized Kähler Manifolds	(local)
December 11	Tommy Wong (UW Madison)	Milnor Fiber of Complex Hyperplane Arrangement.	(local)

Fall Abstracts

Hung Tran

Relative divergence, subgroup distortion, and geodesic divergence

In my presentation, I introduce three new invariants for pairs $(G;H)$ consisting of a finitely generated group $G$ and a subgroup $H$. The first invariant is the upper relative divergence which generalizes Gersten's notion of divergence. The second invariant is the lower relative divergence which generalizes a definition of Cooper-Mihalik. The third invariant is the lower subgroup distortion which parallels the standard notion of subgroup distortion. We examine the relative divergence (both upper and lower) of a group with respect to a normal subgroup or a cyclic subgroup. We also explore relative divergence of $CAT(0)$ groups and relatively hyperbolic groups with respect to various subgroups to better understand geometric properties of these groups. We answer the question of Behrstock and Drutu about the existence of Morse geodesics in $CAT(0)$ spaces with divergence function strictly greater than $r^n$ and strictly less than $r^{n+1}$, where $n$ is an integer greater than $1$. More precisely, we show that for each real number $s>2$, there is a $CAT(0)$ space $X$ with a proper and cocompact action of some finitely generated group such that $X$ contains a Morse bi-infinite geodesic with the divergence equivalent to $r^s$.

Tullia Dymarz

Non-rectifiable Delone sets in amenable groups

In 1998 Burago-Kleiner and McMullen constructed the first examples of coarsely dense and uniformly discrete subsets of R^n that are not biLipschitz equivalent to the standard lattice Z^n. Similarly we find subsets inside the three dimensional solvable Lie group SOL that are not bilipschitz to any lattice in SOL. The techniques involve combining ideas from Burago-Kleiner with quasi-isometric rigidity results from geometric group theory.

Jesse Wolfson

Counting Problems and Homological Stability

In 1969, Arnold showed that the i^{th} homology of the space of un-ordered configurations of n points in the plane becomes independent of n for n>>i. A decade later, Segal extended Arnold's method to show that the i^{th} homology of the space of degree n holomorphic maps from \mathbb{P}^1 to itself also becomes independent of n for large n, and, moreover, that both sequences of spaces have the same limiting homology. We explain how, using Weil's number field/function field dictionary, one might have predicted this topological coincidence from easily verifiable statements about specific counting problems. We then discuss ongoing joint work with Benson Farb and Melanie Wood in which we use other counting problems to predict and discover new instances of homological stability in the topology of complex manifolds.

Matthew Cordes

Morse boundaries of geodesic metric spaces

I will introduce a new type of boundary for proper geodesic spaces, called the Morse boundary, that is constructed with equivalence classes of geodesic rays that identify the ``hyperbolic directions" in that space. (A ray is Morse if quasi-geodesics with endpoints on the ray stay bounded distance from the ray.) This boundary is a quasi-isometry invariant and a visibility space. In the case of a proper CAT(0) space the Morse boundary generalizes the contracting boundary of Charney and Sultan and in the case of a proper Gromov hyperbolic space this boundary is the Gromov boundary. Time permitting I will also discuss some results on Morse boundary of the mapping class group and briefly describe joint work with David Hume developing a capacity dimension for the Morse boundary.

Anton Izosimov

TBA

Jacob Bernstein

Hypersurfaces of low entropy

The entropy is a quantity introduced by Colding and Minicozzi and may be thought of as a rough measure of the geometric complexity of a hypersurface of Euclidean space. It is closely related to the mean curvature flow. On the one hand, the entropy controls the dynamics of the flow. On the other hand, the mean curvature flow may be used to study the entropy. In this talk I will survey some recent results with Lu Wang that show that hypersurfaces of low entropy really are simple.

Yun Su

Higher-order degrees of hypersurface complements.

Gao Chen

Classification of gravitational instantons

A gravitational instanton is a noncompact complete hyperkahler manifold of real dimension 4 with faster than quadratic curvature decay. In this talk, I will discuss the recent work towards the classification of gravitational instantons. This is a joint work with X. X. Chen.

Dan Cristofaro-Gardiner

Higher-dimensional symplectic embeddings and the Fibonacci staircase

McDuff and Schlenk determined when a four dimensional symplectic ellipsoid can be embedded into a ball, and found that when the ellipsoid is close to round, the answer is given by an infinite staircase determined by the odd-index Fibonacci numbers. I will explain joint work with Richard Hind, showing that a generalization of this holds in all even dimensions.

Danny Ruberman

Configurations of embedded spheres

Configurations of lines in the plane have been studied since antiquity. In recent years, combinatorial methods have been used to decide if a specified incidence relation between certain objects ("lines") and other objects ("points") can be realized by actual points and lines in a projective plane over a field. For the real and complex fields, one can weaken the condition to look for topologically embedded lines (circles in the real case, spheres in the complex case) that meet according to a specified incidence relation. I will explain some joint work with Laura Starkston (Stanford) giving new topological restrictions on the realization of configurations of spheres in the complex projective plane.

Archive of past Geometry seminars

2014-2015: Geometry_and_Topology_Seminar_2014-2015

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