Other area seminars. Our e-mail list. Archive of previous semesters

## Spring 2020

Date | Speaker | Title |
---|---|---|

January 24 | Ryan Budney | Isotopy in dimension 4 |

January 31 | Nick Salter | Framed mapping class groups and strata of abelian differentials |

February 7 | Jacob Russell | Relative hyperbolicity in hierarchically hyperbolic spaces |

February 14 | Colin Adams | Hyperbolicity and Turaev Hyperbolicity of Classical and Virtual Knots |

February 21 | Daniele Alessandrini | Non commutative cluster coordinates for Higher Teichmüller Spaces |

February 28 | Abhijit Champanerkar | Right-angled polyhedra and alternating links |

March 6 | Peter Shalen | Quantitative Mostow Rigidity |

March 27 | Keiko Kawamuro | Detection of overtwisted contact structures, loose transverse links, and virtually loose transverse links |

April 10 | Harrison Bray | Random walks on punctured convex real projective surfaces |

April 24 | Rylee Lyman | Some new CAT(0) free-by-cyclic groups |

May 1 | Lei Chen | Actions of Homeo and Diffeo groups on manifolds |

## Abstracts

May 1

**Title**: Actions of Homeo and Diffeo groups on manifolds

**Abstract**: In this talk, I discuss the general question of how to obstruct and construct group actions on manifolds. I will focus on large groups like Homeo(M) and Diff(M) about how they can act on another manifold N. The main result is an orbit classification theorem, which fully classifies possible orbits. I will also talk about some low dimensional applications and open questions. This is a joint work with Kathryn Mann.

April 24

**Title**:Some new CAT(0) free-by-cyclic groups

**Abstract**:As with fundamental groups of 3-manifolds fibering over the circle, free-by-cyclic groups form a varied and interesting class of groups whose geometry depends in large part on the corresponding monodromy, in this case an outer automorphism of the free group. For example, Hagen and Wise showed that word-hyperbolic free-by-cyclic groups act virtually cospecially on CAT(0) cube complexes, while Gersten found an example of a free-by-cyclic group that cannot be even a subgroup of a CAT(0) group. Gersten's group admits a cyclic hierarchy, an iterated splitting as a graph of groups with free-by-cyclic vertex groups and cyclic edge groups, terminating in Z times Z. By contrast, we show that a large class of free-by-cyclic groups admitting an additional symmetry act geometrically on CAT(0) 2-complexes. Up to taking powers this includes mapping tori of all polynomially-growing palindromic and symmetric automorphisms. A key tool in the proof is our construction of so-called CTs for free products.

**Harrison Bray**, University of Michigan

April 10

**Title**: Random walks onn punctured convex real projective surfaces

**Abstract**: I'll discuss the following result, joint with Giulio Tiozzo: Letting \Gamma be the representation of a punctured hyperbolic surface group in PSL(3,R) acting discretely, properly discontinuously, and with finite co-volume on a properly convex set in the projective plane, we have that hitting measure for a random walk on any \Gamma-orbit with certain moment conditions is singular with respect to the classical Patterson-Sullivan measure. The approach extends and adapts the strategy of Maher-Tiozzo for punctured hyperbolic surfaces. We also prove an essential global shadow lemma for finite volume convex real projective manifolds.

**Keiko Kawamuro**, University of Iowa

March 27

**Title**: Detection of overtwisted contact structures, loose transverse links, and virtually loose transverse links

**Abstract**: In this talk, I study punctured abstract open books. Punctures give rise to a closed braid, that is identified with a transverse link in the supported contact structure via the Giroux correspondence. I will define a twist-left-veering open book and show that it supports an overtwisted contact structure. I will also show when the transverse links is loose (the compliment is overtwisted) and virtually loose. This is joint work with Tetsuya Ito.

March 6

**Title**: Quantitative Mostow Rigidity

**Abstract**: The Mostow Rigidity Theorem implies that the geometry of compact hyperbolic manifolds of dimension at least 3 is entirely determined by their topology. This means that any geometric invariant of a hyperbolic manifold may be regarded as a topological invariant. The theorem itself says nothing about the question of how these topological invariants are related to more classical topological invariants of a space. I will describe some progress on this question, which has been a focus of my research for many years. I will emphasize the connection between the volume of a hyperbolic 3-manifold M, which is its most important geometrically defined invariant, and the dimension of the vector space H_1(M,F_p) for certain primes p, which are its most classical computable topologically defined invariants. This includes joint work with Phil Wagreich, Marc Culler, Gilbert Baumslag, Ian Agol, and Jason DeBlois, and some very recent work with Rosemary Guzman.

February 28

**Title**: Right-angled polyhedra and alternating links

**Abstract**: To any prime alternating link, we associate a collection of hyperbolic right-angled ideal polyhedra by relating geometric, topological and combinatorial methods to decompose the link complement. The sum of the hyperbolic volumes of these polyhedra is a new geometric link invariant, which we call the right-angled volume of the alternating link. We give an explicit procedure to compute the right-angled volume from any alternating link diagram, and prove that it is a new lower bound for the hyperbolic volume of the link. This is joing work with Ilya Kofman and Jessica Purcell.

**Daniele Alessandrini**, Columbia

February 21

**Title**:Non commutative cluster coordinates for Higher Teichmüller Spaces

**Abstract**: In higher Teichmuller theory we study subsets of the character varieties

e.g. the Hitchin components and the spaces of maximal representations.

Fock-Goncharov generalized Thurston's shear coordinates and Penner's

Lambda-lengths to the Hitchin components, showing that they have a

beautiful structure of cluster variety.

Here we apply similar ideas to Maximal Representations and we find new

coordinates on these spaces that give them a structure of non-commutative

cluster varieties, in the sense defined by Berenstein-Rethak.

This is joint work with Guichard, Rogozinnikov and Wienhard.

February 14

**Title**: Hyperbolicity and Turaev Hyperbolicity of Classical and Virtual Knots

**Abstract**: We extend the theory of hyperbolicity of links in the 3-sphere to tg-hyperbolicity of virtual links, using the fact that the theory of virtual links can be translated into the theory of links living in closed orientable thickened surfaces. When the boundary surfaces are taken to be totally geodesic, we obtain a tg-hyperbolic structure with a unique associated volume. We will discuss what is known about this invariant. We further employ a construction of Turaev to associate a family of hyperbolic 3-manifolds of finite volume to any classical or virtual link, even if non-hyperbolic. These are in turn used to define the Turaev volume of a link, which is the minimal volume among all the hyperbolic 3-manifolds associated via this Turaev construction. We will talk about what is known.

February 7

**Title**: Relative hyperbolicity in hierarchically hyperbolic spaces

**Abstract**: Relative hyperbolicity and thickness describe incompatible ways that the non-negatively curved parts of a metric space can be organized. In several classes of spaces (Teichmuller space, Coxeter groups, 3-manifold groups) there exists a strict dichotomy between relative hyperbolicity and thickness that produces strong geometric consequences. Behrstock, Drutu, and Mosher have thus asked for which additional classes of spaces can such a dichotomy be established. We investigate this question in the class of hierarchically hyperbolic spaces and produce a combinatorial criteria for detecting relative hyperbolicity. We apply this criteria to prove the separating curve graph of a surface has the relatively hyperbolic versus thick dichotomy.

**Nick Salter**, Columbia University

**Ryan Budney**, University of Victoria

January 24

**Title**: Isotopy in dimension 4

**Abstract**: I will describe why the trivial knot S^{2}-->S^{4} has non-unique spanning discs up to isotopy. This comes from a chain of deductions that include a description of the low-dimensional homotopy-groups of embeddings of S^{1} in S^{1}xS^{n}^{ }(for n>2), a group structure on the isotopy-classes of reducing discs of S^{1}xD^{n}, and the action of the diffeomorphism group Diff(S^{1}xS^{n}) on the embedding space Emb(S^{1}, S^{1}xS^{n}). Roughly speaking, these results say there is no direct translation from dimension 3 to 4, for the Hatcher-Ivanov theorems on spaces of incompressible surfaces.

# Other relevant information.

## Previous semesters:

Fall 2019, Spring 2019, Fall 2018, Spring 2018, Fall 2017, Spring 2017, Fall 2016, Spring 2016, Fall 2015, Spring 2015, Fall 2014, Spring 2014, Fall 2013, Spring 2013, Fall 2012, Spring 2012, Fall 2011, 2010/11, Spring 2010, Fall 2009, Spring 2009, Fall 2008, Spring 2008, Fall 2007, Spring 2007, Fall 2006.## Other area seminars.

- Columbia Symplectic Geometry/Gauge Theory Seminar
- All Columbia Math Dept Seminars
- CUNY Geometry and Topology Seminar
- CUNY Complex Analysis & Dynamics Seminar
- CUNY Magnus Seminar
- Princeton Topology Seminar.