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== Fall 2010 ==
The [[Geometry and Topology]] seminar meets in room '''901 of Van Vleck Hall''' on '''Fridays''' from '''1:20pm - 2:10pm'''.
<br>
For more information, contact Shaosai Huang.


The seminar will be held  in room B901 of Van Vleck Hall on Fridays from 1:20pm - 2:10pm
[[Image:Hawk.jpg|thumb|300px]]
 
 
== Spring 2020 ==


{| cellpadding="8"
{| cellpadding="8"
Line 9: Line 14:
!align="left" | host(s)
!align="left" | host(s)
|-
|-
|September 10
|Feb. 7
|[http://www.math.wisc.edu/~oh/ Yong-Geun Oh] (UW Madison)
|Xiangdong Xie  (Bowling Green University)
|[[#Yong-Geun Oh (UW Madison)|
| Minicourse 1: Quasiconformal structure on the ideal boundary of Gromov hyperbolic spaces
''Counting embedded curves in Calabi-Yau threefolds and Gopakumar-Vafa invariants'']]
|(Dymarz)
|local
|-
|September 17
|Leva Buhovsky (U of Chicago)
|[[#Leva Buhovsky (U of Chicago)|
''On the uniqueness of Hofer's geometry'']]
|[http://www.math.wisc.edu/~oh/ Yong-Geun]
|-
|September 24
|[http://sites.google.com/site/polterov/home/ Leonid Polterovich] (Tel Aviv U and U of Chicago)
|[[#Leonid Polterovich (Tel Aviv U and U of Chicago)|
''Poisson brackets and symplectic invariants'']]
|[http://www.math.wisc.edu/~oh/ Yong-Geun]
|-
|-
|October 8
|Feb. 14
|[http://www.math.wisc.edu/~stpaul/ Sean Paul] (UW Madison)
|Xiangdong Xie  (Bowling Green University)
|[[#Sean Paul (UW Madison)|
| Minicourse 2: Quasiconformal structure on the ideal boundary of Gromov hyperbolic spaces
''Canonical Kahler metrics and the stability of projective varieties'']]
|(Dymarz)
|local
|-
|-
|October 15
|Feb. 21
|Conan Leung (Chinese U. of Hong Kong)
|Xiangdong Xie  (Bowling Green University)
|[[#Conan Leung (Chinese U. of Hong Kong)|
| Minicourse 3: Quasiconformal structure on the ideal boundary of Gromov hyperbolic spaces
''SYZ mirror symmetry for toric manifolds'']]
|(Dymarz)
|Honorary fellow, local
|-
|-
|October 22
|Feb. 28
|[http://www.mathi.uni-heidelberg.de/~banagl/ Markus Banagl] (U. Heidelberg)
|Kuang-Ru Wu (Purdue University)
|[[# Markus Banagl (U. Heidelberg)|
|Griffiths extremality, interpolation of norms, and Kahler quantization
''Intersection Space Methods and Their Application to Equivariant Cohomology, String Theory, and Mirror Symmetry'']]
|(Huang)
|[http://www.math.wisc.edu/~maxim/ Maxim]
|-
|-
|October 29
|Mar. 6
|[http://www.math.umn.edu/~zhux0086/ Ke Zhu] (U of Minnesota)
|Yuanqi Wang (University of Kansas)
|[[#Ke Zhu (U of Minnesota)|
|Moduli space of G2−instantons on 7−dimensional product manifolds
''Thick-thin decomposition of Floer trajectories and adiabatic gluing'']]
|(Huang)
|[http://www.math.wisc.edu/~oh/ Yong-Geun]
|-
|-
|November 5
|Mar. 13 <b>CANCELED</b>
|[http://www.math.psu.edu/tabachni/ Sergei Tabachnikov]  (Penn State)
|Karin Melnick (University of Maryland)
|[[#Sergei Tabachnikov (Penn State)|
|A D'Ambra Theorem in conformal Lorentzian geometry
''Algebra, geometry, and dynamics of the pentagram map'']]
|(Dymarz)
|[http://www.math.wisc.edu/~maribeff/ Gloria]
|-
|November 19
|Ma Chit (Chinese U. of Hong Kong)
|[[#Ma Chit (Chinese U. of Hong Kong)|
''A growth estimate of lattice points in Gorenstein cones using toric Einstein metrics'']]
|Graduate student, local
|-
|December 3
|[http://www.math.northwestern.edu/~zaslow/ Eric Zaslow]  (Northwestern University)
|[[#Eric Zaslow (Northwestern University)|
''Ribbon Graphs and Mirror Symmetry'']]
|[http://www.math.wisc.edu/~oh/ Yong-Geun and Conan Leung]
|-
|December 10
|Wenxuan Lu  (MIT)
|[[#Wenxuan Lu (MIT)|
''Instanton Correction, Wall Crossing And Mirror Symmetry Of Hitchin's Moduli
Spaces'']]
|[http://www.math.wisc.edu/~oh/ Young-Geun and Conan Leung]
|-
|-
|<b>Mar. 25</b> <b>CANCELED</b>
|Joerg Schuermann (University of Muenster, Germany)
|An introduction to Poincare-Hopf theorems for singular spaces via characteristic cycles
|(Maxim)
|-
|-
|Mar. 27 <b>CANCELED</b>
|David Massey (Northeastern University)
|Extracting easily calculable algebraic data from the vanishing cycle complex
|(Maxim)
|-
|-
|<b>Apr. 10</b> <b>CANCELED</b>
|Antoine Song (Berkeley)
|TBA
|(Chen)
|}
|}


 
== Fall 2019 ==
 
== Spring 2011 ==
 
The seminar will be held  in room B901 of Van Vleck Hall on Fridays from 1:20pm - 2:10pm


{| cellpadding="8"
{| cellpadding="8"
Line 92: Line 68:
!align="left" | host(s)
!align="left" | host(s)
|-
|-
|January 21
|Oct. 4
|Mohammed Abouzaid (Clay Institute & MIT)
|Ruobing Zhang (Stony Brook University)
|[[#Mohammed Abouzaid (Clay Institute & MIT)|
| Geometric analysis of collapsing Calabi-Yau spaces
''TBA'']]
|(Chen)
|[http://www.math.wisc.edu/~oh/ Yong-Geun]
|-
|-
|-
|March 4
|Oct. 25
|[http://www.massey.math.neu.edu/ David Massey] (Northeastern)
|Emily Stark (Utah)
|[[#David Massey (Northeastern)|
| Action rigidity for free products of hyperbolic manifold groups
''TBA'']]
|(Dymarz)
|[http://www.math.wisc.edu/~maxim/ Maxim]
|-
|-
|March 11
|Nov. 8
|Danny Calegari (Cal Tech))
|Max Forester (University of Oklahoma)
|[[#Danny Calegari (Cal Tech)|
|Spectral gaps for stable commutator length in some cubulated groups
''TBA'']]
|(Dymarz)
|[http://www.math.wisc.edu/~oh/ Yong-Geun]
|-
|-
|May 6
|Nov. 22
|[http://www.math.neu.edu/~suciu/ Alex Suciu] (Northeastern)
|Yu Li (Stony Brook University)
|[[#Alex Suciu (Northeastern)|
|On the structure of Ricci shrinkers
''TBA'']]
|(Huang)
|[http://www.math.wisc.edu/~maxim/ Maxim]
|-
|-
|}
|}


== Abstracts ==
==Spring Abstracts==
===Yong-Geun Oh (UW Madison)===
''Counting embedded curves in Calabi-Yau threefolds and Gopakumar-Vafa invariants''


Gopakumar-Vafa BPS invariant is some integer counting invariant of the cohomology
===Xiangdong Xie===
of D-brane moduli spaces in string theory. In relation to the Gromov-Witten theory,
it is expected that the invariant would coincide with the `number' of embedded
(pseudo)holomorphic curves (Gopakumar-Vafa conjecture). In this talk, we will explain the speaker's recent
result that the latter integer invariants can be defined for a generic choice of
compatible almost complex structures. We will also discuss the corresponding
wall-crossing phenomena and some open questions towards a complete solution to
the Gopakumar-Vafa conjecture.


===Leva Buhovsky (U of Chicago)===
The quasiconformal structure on the ideal boundary of Gromov hyperbolic spaces has played
''On the uniqueness of Hofer's geometry''
an important role in various  rigidity questions in geometry and group theory.
In these talks I  shall give an introduction to this topic.  In the first talk I will introduce Gromov hyperbolic spaces, define their ideal boundary,  and  discuss their basic properties.  In the second and third talks I will define the visual metrics on the ideal boundary, explain the connection between quasiisometries of   Gromov hyperbolic space and quasiconformal maps on  their ideal boundary, and indicate  how the quasiconformal structure on the ideal boundary can be used to deduce rigidity.


In this talk we address the question whether Hofer's metric is unique among the Finsler-type bi-invariant metrics on the group of Hamiltonian diffeomorphisms. The talk is based on a recent joint work with Yaron Ostrover.
===Kuang-Ru Wu===


===Leonid Polterovich (Tel Aviv U and U of Chicago)===
Following Kobayashi, we consider Griffiths negative complex Finsler bundles, naturally leading us to introduce Griffiths extremal Finsler metrics. As we point out, this notion is closely related to the theory of interpolation of norms, and is characterized by an equation of complex Monge– Ampere type, whose corresponding Dirichlet problem we solve. As applications, we prove that Griffiths extremal Finsler metrics quantize solutions to a natural PDE in Kahler geometry, related to the construction of flat maps for the Mabuchi metric. This is joint work with Tamas Darvas.
''Poisson brackets and symplectic invariants''


We discuss new invariants associated to collections of closed subsets
===Yuanqi Wang===
of a symplectic manifold. These invariants are defined
$G_{2}-$instantons are 7-dimensional analogues of flat connections in dimension 3. It is part of Donaldson-Thomas’ program to generalize the fruitful gauge theory in dimensions 2,3,4 to dimensions 6,7,8. The moduli space of  $G_{2}-$instantons, with virtual dimension $0$, is   expected to have interesting  geometric structure and yield enumerative invariant for the underlying $7-$dimensional manifold.  
through an elementary variational problem involving Poisson brackets.
The proof of non-triviality of these invariants requires methods of modern
symplectic topology (Floer theory). We present applications
to approximation theory on symplectic manifolds and to Hamiltonian dynamics.
The talk is based on a work in progress with Lev Buhovsky and Michael Entov.


===Sean Paul (UW Madison)===
In this talk, in some reasonable special cases and a fairly complete manner, we will describe the relation between the moduli space of $G_{2}-$instantons and an algebraic geometry moduli on a Calabi-Yau 3-fold.
''Canonical Kahler metrics and the stability of projective varieties"


I will give a survey of my own work on this problem, the basic reference is:
===Karin Melnick===
http://arxiv.org/pdf/0811.2548v3


===Conan Leung (Chinese U. of Hong Kong)===
D'Ambra proved in 1988 that the isometry group of a compact, simply connected, real-analytic Lorentzian manifold must be compact. I will discuss my recent theorem that the conformal group of such a manifold must also be compact, and how it relates to the Lorentzian Lichnerowicz Conjecture.
''SYZ mirror symmetry for toric manifolds''


===Markus Banagl (U. Heidelberg)===
===Joerg Schuermann===
''Intersection Space Methods and Their Application to Equivariant Cohomology, String Theory, and Mirror Symmetry.''


Using homotopy theoretic methods, we shall associate to certain classes of
We give an introduction to Poincare-Hopf theorems for singular spaces via characteristic cycles, based on stratified Morse theory for constructible functions. The corresponding local index of an isolated critical point (in a stratified sense) of a one-form depends on the constructible function, specializing for different choices to well known indices like the radial, GSV or Euler obstruction index.
singular spaces generalized geometric Poincaré complexes called intersection
spaces. Their cohomology is generally not isomorphic to intersection
cohomology.
In this talk, we shall concentrate on the applications of the new
cohomology theory to the equivariant real cohomology of isometric actions of
torsionfree discrete groups, to type II string theory and D-branes, and to
the relation of the new theory to classical intersection cohomology under
mirror symmetry.


===Ke Zhu (U of Minnesota)===
===David Massey===
''Thick-thin decomposition of Floer trajectories and adiabatic gluing''


Let f be a generic Morse function on a symplectic manifold M.
Given a complex analytic function on an open subset U  of C<sup>n+1</sup>, one may consider the complex of sheaves of vanishing cycles along f of the constant sheaf Z<sub>U</sub>. This complex encodes on the cohomological level the reduced cohomology of the Milnor fibers of f at each of f<sup>-1</sup>(0). The question is: how does one calculate (ideally, by hand)  any useful numbers about this vanishing cycle complex? One answer is to look at the Lê numbers of f. We will discuss the precise relationship between these objects/numbers.
For Floer trajectories of Hamiltonian \e f, as \e goes to 0 Oh proved that
they converge to “pearl complex” consisiting of J-holomorphic spheres
and joining gradient segments of f. The J-holomorphic spheres come from the  
“thick” part of Floer trajectories and the gradient segments come from
the “thin” part. Similar “thick-thin” compactification result has
also been obtained by Mundet-Tian in twisted holomorphic map setting. In
this talk, we prove the reverse gluing result in the simplest setting: we
glue from disk-flow-dsik configurations to nearby Floer trajectories of
Hamitonians K_{\e} for sufficiently small \e and also show the  
surjectivity. (Most part of the Hamiltonian K_{\e} is \ef). We will discuss  
the application to PSS isomorphism. This is a joint work with Yong-Geun Oh.


===Sergei Tabachnikov (Penn State)===
===Antoine Song===
''Algebra, geometry, and dynamics of the pentagram map''


Introduced by R. Schwartz almost 20  years ago, the pentagram map acts on plane n-gons, considered up to projective equivalence, by drawing the diagonals that connect second-nearest vertices and taking the new n-gon formed by their intersections. I shall survey recent work on the pentagram map, in particular, I shall demonstrate  that the dynamics of the pentagram map  is completely integrable. I shall also explain that the pentagram map is a discretization of the Boussinesq equation, a well known completely integrable partial differential equation. A surprising relation between the spaces of polygons and combinatorial objects called the 2-frieze patterns (generalizing the frieze patterns of Coxeter) will be described. Eight new(?) configuration theorems of projective geometry will be demonstrated. The talk is illustrated by computer animation.
TBA


===Ma Chit (Chinese U. of Hong Kong)===
==Fall Abstracts==
''A growth estimate of lattice points in Gorenstein cones using toric Einstein metrics''


Using the existence of Einstein metrics on toric Kahler and Sasaki manifolds, a lower bound estimate on the growth of lattice points is obtained for Gorenstein cones. This talk is based on a joint work with Conan Leung. 
===Ruobing Zhang===


===Eric Zaslow (Northwestern University)===
This talk centers on the degenerations of Calabi-Yau metrics. We will focus on the interactions between algebraic degenerations and metric convergence with highly singular behaviors in the collapsing case. As the complex structures degenerate, the collapsing Calabi-Yau metrics may exhibit various wild geometric properties with highly non-algebraic features.
''Ribbon Graphs and Mirror Symmetry''


I will define, for each ribbon graph, a dg category,
First, as motivating examples, we will describe our recent results on the new collapsing mechanisms of K3 surfaces. Next, we will switch to higher dimensions and we will exhibit some entirely new constructions of degenerating Calabi-Yau metrics which are expected to work in broader contexts. Complex structures degeneration will be accurately characterized by the bubbling and singularity analysis in a geometric manner.
and explain the conjectural relation to mirror symmetry.
I will being by reviewing how T-duality relates
coherent sheaves on toric varieties to constructible sheaves
on a vector space, then use this relation to glue
toric varieties together.  In one-dimension, the
category of sheaves on such gluings has a
description in terms of ribbon graphs.
These categories are conjecturally
related to the Fukaya category of a noncompact
hypersurface mirror to the variety with toric
components.


I will use very basic examples.
===Emily Stark===
This work is joint with Nicolo' Sibilla
and David Treumann.


The relationship between the large-scale geometry of a group and its algebraic structure can be studied via three notions: a group's quasi-isometry class, a group's abstract commensurability class, and geometric actions on proper geodesic metric spaces. A common model geometry for groups G and G' is a proper geodesic metric space on which G and G' act geometrically. A group G is action rigid if every group G' that has a common model geometry with G is abstractly commensurable to G. For example, a closed hyperbolic n-manifold group is not action rigid for all n at least three. In contrast, we show that free products of closed hyperbolic manifold groups are action rigid. Consequently, we obtain the first examples of Gromov hyperbolic groups that are quasi-isometric but do not virtually have a common model geometry. This is joint work with Daniel Woodhouse.


===Wenxuan Lu (MIT)===
===Max Forester===
''Instanton Correction, Wall Crossing And Mirror Symmetry Of Hitchin's Moduli
Spaces''


We study two instanton correction problems of Hitchin's moduli spaces along with
I will discuss stable commutator length (scl) in groups, and some gap theorems for the scl spectrum. Such results say that for various groups, scl of an element is always either zero or is larger than some uniform constant. I will discuss the cases of right-angled Artin groups and certain right-angled Coxeter groups. This is joint work with Pallavi Dani, Ignat Soroko, and Jing Tao.
their wall crossing formulas. The hyperkahler metric of a Hitchin's moduli space
can be put into an instanton-corrected form according to physicists Gaiotto,
Moore and Neitzke. The problem boils down to the construction of a set of
special coordinates which can be constructed as Fock-Goncharov coordinates
associated with foliations of quadratic differentials on a Riemann surface. A
wall crossing formula of Kontsevich and Soibelman arises both as a crucial
consistency condition and an effective computational tool. On the other hand
Gross and Siebert have succeeded in determining instanton corrections of
complex structures of Calabi-Yau varieties in the context of mirror symmetry
from a singular affine structure with additional data.  We will show that the
two instanton correction problems are equivalent in an appropriate sense. This
is a nontrivial statement of mirror symmetry of Hitchin's moduli spaces which
till now has been mostly studied in the framework of geometric Langlands
duality.  This result provides examples of Calabi-Yau varieties where the
instanton correction (in the sense of mirror symmetry) of  metrics and complex
structures can be determined.


===Mohammed Abouzaid (Clay Institute & MIT)===
===Yu Li===
''TBA''
We develop a structure theory for non-collapsed Ricci shrinkers without any curvature condition. As an application, we show that any Ricci shrinker whose second eigenvalue of the curvature operator is positive must be a quotient of sphere.


===Danny Calegari (Cal Tech)===
== Archive of past Geometry seminars ==
''TBA''
2018-2019  [[Geometry_and_Topology_Seminar_2018-2019]]
<br><br>
2017-2018 [[Geometry_and_Topology_Seminar_2017-2018]]
<br><br>
2016-2017  [[Geometry_and_Topology_Seminar_2016-2017]]
<br><br>
2015-2016: [[Geometry_and_Topology_Seminar_2015-2016]]
<br><br>
2014-2015: [[Geometry_and_Topology_Seminar_2014-2015]]
<br><br>
2013-2014: [[Geometry_and_Topology_Seminar_2013-2014]]
<br><br>
2012-2013: [[Geometry_and_Topology_Seminar_2012-2013]]
<br><br>
2011-2012: [[Geometry_and_Topology_Seminar_2011-2012]]
<br><br>
2010: [[Fall-2010-Geometry-Topology]]

Latest revision as of 18:56, 3 September 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 Shaosai Huang.

Hawk.jpg


Spring 2020

date speaker title host(s)
Feb. 7 Xiangdong Xie (Bowling Green University) Minicourse 1: Quasiconformal structure on the ideal boundary of Gromov hyperbolic spaces (Dymarz)
Feb. 14 Xiangdong Xie (Bowling Green University) Minicourse 2: Quasiconformal structure on the ideal boundary of Gromov hyperbolic spaces (Dymarz)
Feb. 21 Xiangdong Xie (Bowling Green University) Minicourse 3: Quasiconformal structure on the ideal boundary of Gromov hyperbolic spaces (Dymarz)
Feb. 28 Kuang-Ru Wu (Purdue University) Griffiths extremality, interpolation of norms, and Kahler quantization (Huang)
Mar. 6 Yuanqi Wang (University of Kansas) Moduli space of G2−instantons on 7−dimensional product manifolds (Huang)
Mar. 13 CANCELED Karin Melnick (University of Maryland) A D'Ambra Theorem in conformal Lorentzian geometry (Dymarz)
Mar. 25 CANCELED Joerg Schuermann (University of Muenster, Germany) An introduction to Poincare-Hopf theorems for singular spaces via characteristic cycles (Maxim)
Mar. 27 CANCELED David Massey (Northeastern University) Extracting easily calculable algebraic data from the vanishing cycle complex (Maxim)
Apr. 10 CANCELED Antoine Song (Berkeley) TBA (Chen)

Fall 2019

date speaker title host(s)
Oct. 4 Ruobing Zhang (Stony Brook University) Geometric analysis of collapsing Calabi-Yau spaces (Chen)
Oct. 25 Emily Stark (Utah) Action rigidity for free products of hyperbolic manifold groups (Dymarz)
Nov. 8 Max Forester (University of Oklahoma) Spectral gaps for stable commutator length in some cubulated groups (Dymarz)
Nov. 22 Yu Li (Stony Brook University) On the structure of Ricci shrinkers (Huang)

Spring Abstracts

Xiangdong Xie

The quasiconformal structure on the ideal boundary of Gromov hyperbolic spaces has played an important role in various rigidity questions in geometry and group theory. In these talks I shall give an introduction to this topic. In the first talk I will introduce Gromov hyperbolic spaces, define their ideal boundary, and discuss their basic properties. In the second and third talks I will define the visual metrics on the ideal boundary, explain the connection between quasiisometries of Gromov hyperbolic space and quasiconformal maps on their ideal boundary, and indicate how the quasiconformal structure on the ideal boundary can be used to deduce rigidity.

Kuang-Ru Wu

Following Kobayashi, we consider Griffiths negative complex Finsler bundles, naturally leading us to introduce Griffiths extremal Finsler metrics. As we point out, this notion is closely related to the theory of interpolation of norms, and is characterized by an equation of complex Monge– Ampere type, whose corresponding Dirichlet problem we solve. As applications, we prove that Griffiths extremal Finsler metrics quantize solutions to a natural PDE in Kahler geometry, related to the construction of flat maps for the Mabuchi metric. This is joint work with Tamas Darvas.

Yuanqi Wang

$G_{2}-$instantons are 7-dimensional analogues of flat connections in dimension 3. It is part of Donaldson-Thomas’ program to generalize the fruitful gauge theory in dimensions 2,3,4 to dimensions 6,7,8. The moduli space of $G_{2}-$instantons, with virtual dimension $0$, is expected to have interesting geometric structure and yield enumerative invariant for the underlying $7-$dimensional manifold.

In this talk, in some reasonable special cases and a fairly complete manner, we will describe the relation between the moduli space of $G_{2}-$instantons and an algebraic geometry moduli on a Calabi-Yau 3-fold.

Karin Melnick

D'Ambra proved in 1988 that the isometry group of a compact, simply connected, real-analytic Lorentzian manifold must be compact. I will discuss my recent theorem that the conformal group of such a manifold must also be compact, and how it relates to the Lorentzian Lichnerowicz Conjecture.

Joerg Schuermann

We give an introduction to Poincare-Hopf theorems for singular spaces via characteristic cycles, based on stratified Morse theory for constructible functions. The corresponding local index of an isolated critical point (in a stratified sense) of a one-form depends on the constructible function, specializing for different choices to well known indices like the radial, GSV or Euler obstruction index.

David Massey

Given a complex analytic function on an open subset U of Cn+1, one may consider the complex of sheaves of vanishing cycles along f of the constant sheaf ZU. This complex encodes on the cohomological level the reduced cohomology of the Milnor fibers of f at each of f-1(0). The question is: how does one calculate (ideally, by hand) any useful numbers about this vanishing cycle complex? One answer is to look at the Lê numbers of f. We will discuss the precise relationship between these objects/numbers.

Antoine Song

TBA

Fall Abstracts

Ruobing Zhang

This talk centers on the degenerations of Calabi-Yau metrics. We will focus on the interactions between algebraic degenerations and metric convergence with highly singular behaviors in the collapsing case. As the complex structures degenerate, the collapsing Calabi-Yau metrics may exhibit various wild geometric properties with highly non-algebraic features.

First, as motivating examples, we will describe our recent results on the new collapsing mechanisms of K3 surfaces. Next, we will switch to higher dimensions and we will exhibit some entirely new constructions of degenerating Calabi-Yau metrics which are expected to work in broader contexts. Complex structures degeneration will be accurately characterized by the bubbling and singularity analysis in a geometric manner.

Emily Stark

The relationship between the large-scale geometry of a group and its algebraic structure can be studied via three notions: a group's quasi-isometry class, a group's abstract commensurability class, and geometric actions on proper geodesic metric spaces. A common model geometry for groups G and G' is a proper geodesic metric space on which G and G' act geometrically. A group G is action rigid if every group G' that has a common model geometry with G is abstractly commensurable to G. For example, a closed hyperbolic n-manifold group is not action rigid for all n at least three. In contrast, we show that free products of closed hyperbolic manifold groups are action rigid. Consequently, we obtain the first examples of Gromov hyperbolic groups that are quasi-isometric but do not virtually have a common model geometry. This is joint work with Daniel Woodhouse.

Max Forester

I will discuss stable commutator length (scl) in groups, and some gap theorems for the scl spectrum. Such results say that for various groups, scl of an element is always either zero or is larger than some uniform constant. I will discuss the cases of right-angled Artin groups and certain right-angled Coxeter groups. This is joint work with Pallavi Dani, Ignat Soroko, and Jing Tao.

Yu Li

We develop a structure theory for non-collapsed Ricci shrinkers without any curvature condition. As an application, we show that any Ricci shrinker whose second eigenvalue of the curvature operator is positive must be a quotient of sphere.

Archive of past Geometry seminars

2018-2019 Geometry_and_Topology_Seminar_2018-2019

2017-2018 Geometry_and_Topology_Seminar_2017-2018

2016-2017 Geometry_and_Topology_Seminar_2016-2017

2015-2016: Geometry_and_Topology_Seminar_2015-2016

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

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