# 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.

• Friday February 23, 2018 at 15:30, Wachman 617
Planar open books and singularities

Olga Plamenevskaya, SUNY Stony Brook

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

Abstract: Due to work of Giroux, contact structures on 3-manifolds can be topologically described by their open books decompositions (which in turn can be encoded via fibered links). A contact structure is called planar if it admits an open book with fibers of genus 0. Symplectic fillings of such contact structures can be understood, by a theorem of Wendl, via Lefschetz fibrations with the same planar fiber. Using this together with topological considerations, we prove a new obstruction to planarity (in terms of intersection form of fillings) and obtain a few corollaries. In particular, we consider contact structures that arise in a canonical way on links of surface singularities, and show that the canonical contact structure on the link is planar if and only if the singularity is rational. (Joint work with P. Ghiggini and M. Golla.)

In the background talk (11:00 AM), I will discuss topological properties of Lefschetz fibrations over a disk, focusing on the case where fiber is a surface of genus 0. The boundary of the 4-manifold given by Lefschetz fibration has an induced open book and a contact structure. This will be the setting for my second talk.

• Friday February 23, 2018 at 14:00, Wachman 617
Understanding quantum link invariants via surfaces in 3-manifolds

Christine Lee, University of Texas

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

Abstract: Quantum link invariants lie at the intersection of hyperbolic geometry, 3-dimensional manifolds, quantum physics, and representation theory, where a central goal is to understand its connection to other invariants of links and 3-manifolds. In this talk, we will introduce the colored Jones polynomial, an important example of quantum link invariants. We will discuss how studying properly embedded surfaces in a 3-manifold provides insight into the topological and geometric content of the polynomial. In particular, we will describe how relating the definition of the polynomial to surfaces in the complement of a link shows that it determines boundary slopes and bounds the hyperbolic volume of many links, and we will explore the implication of this approach on these classical invariants.

In the background talk (9:30, AM) I'll introduce the colored Jones polynomial and discuss the many conjectures/open problems surrounding the polynomial, to give the research talk more context.

• Wednesday February 21, 2018 at 14:30, Wachman 617
Some hyperbolic actions of subgroups of Aut(F_n)

Lee Mosher, Rutgers Newark

In the course of our theorem on the $H^2_b$-alternative for $Out(F_n)$ — every finitely generated subgroup of $Out(F_n)$ is either virtually abelian or has second bounded cohomology of uncountable dimension — the case of subgroups of natural embeddings of $Aut(F_k)$ into $Out(F_n)$ led us to subgroups of $Aut(F_k)$ which have interesting new hyperbolic actions arising from “suspension” constructions, generalizing a thread of hyperbolic suspension constructions which goes back to a theorem of W. Thurston. In this talk we will describe these suspension constructions, and we will speculate on what may unify them.

This is joint work with Michael Handel.

• Wednesday February 14, 2018 at 14:45, Wachman 617
Moduli of Curves: from GT theory to Arithmetic Geometry

Benjamin Collas, Bayreuth

The goal of Grothendieck-Teichmüller theory is to lead an arithmetic study of the moduli spaces of curves via their geometric fundamental group. Once identified to the profinite orbifold fundamental group, the latter provides a computational framework in terms of braid and mapping class groups.
While the classical GT theory, as developed by Drinfel'd, Lochak, Nakamura, Schneps et al., essentially deals with the schematic or topological properties of the spaces at infinity'', the moduli spaces of curves also admit a stack or orbifold structure that encodes the automorphisms of curves. The goal of this talk is to show how fundamental group theoretic properties of the mapping class groups and Hatcher-Thurston pants decomposition lead to orbifold arithmetic results, then to potential finer GT groups.
We will present in detail this analytic Teichmüller approach and indicate the essential obstacles encountered, before briefly explaining how they can be circumvent in terms of arithmetic geometry.

• Wednesday February 7, 2018 at 14:30, Wachman 617
On the rank of hyperbolic group extensions

Sam Taylor, Temple University

The rank of a group is the minimal cardinality of a generating set. While simple to define, this quantity is notoriously difficult to calculate, even for well behaved groups. In this talk, I’ll introduce some algebraic and geometric properties of hyperbolic group extensions and discuss how their bundle structure can be used to understand rank in this setting.

• Wednesday January 31, 2018 at 14:30, Wachman 617
Distances between hyperbolic tubes

David Futer, Temple University

Abstract: The study of hyperbolic manifolds often begins with the thick-thin decomposition. Given a number $\epsilon > 0$, we decompose a manifold into the $\epsilon$-thin part (points on essential loops of length less than $\epsilon$), and the $\epsilon$-thick part (everything else). The Margulis lemma says that there is a universal number $\epsilon_n$, depending only on the dimension, such that the thin part of every hyperbolic $n$-manifold has very simple topology.

In dimension 3, we still do not know the optimal Margulis constant $\epsilon_3$. Part of the problem is that while the topology is simple, the geometry of $\epsilon$-thin tubes can be quite complicated. I will describe some results that control and estimate the geometry, which has applications to narrowing down the value of the Margulis constant. This is joint work with Jessica Purcell and Saul Schleimer.