University of California, Santa Barbara Topology Seminar

The topology seminar is co-organized by myself (Catherine Pfaff) and Priyam Patel. Due to a scheduling conflict, Priyam is the primary organizer during the Winter 2017 quarter, so please contact her if you would like to give a talk in the seminar.

Directions and maps for UCSB and the Mathematics Department.

KITP's guide to local accommodation

Time: Tu 3:30-4:30 (except as noted)

Place: South Hall 4607(B) (except as noted)



10/4/2016, Mark Hagen (Cambridge)

Title: Acylindrical hyperbolicity from cubical small-cancellation

Abstract: I will define cubical presentations and cubical small-cancellation groups, in the sense of Wise, and explain the increased flexibility this offers compared to classical small-cancellation theory. Then I'll talk about associating hyperbolic graphs to CAT(0) cube complexes, and how to use this to define a hyperbolic space on which a cubical small-cancellation group acts "nicely". From this, one can show that cubical small-cancellation groups are acylindrically hyperbolic, which has various consequences. This result generalizes the fact that strong classical small-cancellation implies hyperbolicity and is closely related to a recent result of Gruber-Sisto on acylindrical hyperbolicity of "graphical" small-cancellation groups. This talk is mainly about recent joint work with Goulnara Arzhantseva.

10/11/2016, 16:15, V4-116: Ilya Kapovich (UIUC)

Title: Cannon-Thurston maps for hyperbolic free group extensions

Abstract: Let FN be a free group of finite rank N ≥ 3 and let Γ ≤ Out(FN) be a finitely generated "convex cocompact" subgroup, that is, such that the orbit map from Γ to the free factor complex of FN is a quasi-isometric embedding. Assume also that Γ is purely atoroidal. In this case Γ determines an extension group EΓ of Γ with the quotient EΓ/FN=Γ, and it is known by a result of Dowdall and Taylor that the group EΓ is then word-hyperbolic. By a general result of Mitra the inclusion of FN in EΓ extends to a continuous surjective FN-equivariant map between their hyperbolic boundaries j: ∂ FN → ∂ EΓ, called the Cannon-Thurston map. We analyze the structure of this map and prove that the map is finite-to-one, with multiplicity at most 2N. The talk is based on a joint paper with Spencer Dowdall and Sam Taylor.

10/18/2016, Nancy Scherich (UCSB)

Title: Mapping the Braid Groups into Lattices

Abstract: Many well loved representations of the braid groups have the property that the image matrices fix a Hermitian form. Using this form, one can show that specializing to certain Salem numbers places the image inside a lattice. While it is unknown whether the image itself is a lattice, there are some interesting results on commensurability of the target lattices.


11/1/2016, Jing Tao (University of Oklahoma)

Title: Effective quasimorphisms on right-angled Artin groups

Abstract: I will discuss the construction of a large family of homogeneous quasimorphisms on right-angled Artin groups, which give rise to a lower bound of 1/24 for the stable commutator length in these groups. This work is joint with Talia Fernos and Max Forester.

11/8/2016, Mark Bell (UIUC)

Title: Nielsen--Thurston classification via the curve graph

Abstract: We will discuss a new polynomial-time algorithm, joint with Richard Webb, for computing the Nielsen--Thurston type of a mapping class. The procedure works by considering the maps action on the curve graph, which records the pairs of essential closed curves that are disjoint.

To be able to compute this action, we need to be able to construct geodesics through the curve graph. However, this graph is locally infinite and so standard pathfinding algorithms struggle. We will discuss a new refinement of the techniques of Leasure, Shackleton, Watanabe and Webb for overcoming this local infiniteness that allows such geodesics to be found in polynomial time (in terms of their length).

11/15/2016, Spencer Dowdall (Vanderbilt)

Title: Hyperbolicity in Outer space with applications to free group extensions

Abstract: The "Outer space" of the rank n free group F_n is a contractible metric space on which the Outer automorphism group Out(F_n) acts properly discontinuously. It was introduced by Culler and Vogtmann in 1986 and is now an important tool for the topological and geometric study of Out(F_n). This talk will focus on the geometry of Outer space and implications for free group extensions. The first aspects of hyperbolicity in Outer space were discovered by Algom-Kfir, who showed that axes of fully irreducible automorphisms are strongly contracting. In this talk I will present a characterization of this strongly contracting property in terms of the geodesic's projection to the free factor complex. This characterization allows one to exploit the hyperbolicity of Outer space to study many geometric aspects of free group extensions. Results here include a flexible means of producing hyperbolic free group extensions, qualitative statements regarding their structure and quasiconvex subgroups, and quantitative results about their Cannon-Thurston maps. Mostly joint with Sam Taylor, and some joint with Ilya Kapovich and Sam Taylor.

11/22/2016, Robert Kropholler (Tufts)

Title: Finiteness properties and free Abelian groups

Abstract: Finiteness properties of groups come in many flavours, I will discuss topological finiteness properties. These relate to the finiteness of skelata in a classifying space. Groups with interesting finiteness properties have been found in many ways, however all such examples contains free Abelian subgroups of high rank. I will discuss some constructions of groups discussing the various ways we can reduce the rank of free Abelian subgroups. I will introduce all the concepts required for my talk.

11/29/2016, Valentina Disarlo (Heidelberg)

Title: On the geometry of the flip graph

Abstract: The flip graph of an orientable punctured surface is the graph whose vertices are the ideal triangulations of the surface (up to isotopy) and whose edges correspond to flips. Its combinatorics is crucial in works of Thurston and Penner’s decorated Teichmuller theory. In this talk we will explore some geometric properties of this graph, in particular we will see that it provides a coarse model of the mapping class group in which the mapping class groups of some subsurfaces are convex. We will also establish upper and lower bounds on the growth of the diameter of the flip graph modulo the mapping class group, extending a result of Sleator-Tarjan-Thurston. This is a joint work with Hugo Parlier.

12/2/2016 (FRIDAY), 2pm SH6637, Jenya Sapir (UIUC)

Title: A Birman-Series result for infinitely self-intersecting geodesics on surfaces

Abstract: Let S be a hyperbolic surface. Birman and Series showed that there is a single nowhere dense, Hausdorff dimension 1 subset of S that contains the images of all complete (bi-infinite) simple geodesics. We examine sets of geodesics with different possible self-intersection rates and show that the Birman-Series type result holds for all of the sets where the rate is o(l2) intersections per length l subarc. In contrast, the set of geodesics with the maximal possible self-intersection rate, which is k l2, is dense and has full Hausdorff dimension in S.

12/6/2016, 2pm, Pallavi Dani (Louisiana State University)

Title: Commensurability classification for certain right-angled Coxeter groups

Abstract: Two finitely generated groups are abstractly commensurable if they have isomorphic finite-index subgroups. Classifying groups up to commensurability is a fundamental question in geometric group theory. I will discuss joint work with Emily Stark and Anne Thomas on the commensurability classification of a class of hyperbolic right-angled Coxeter groups. A key step in our proof is showing that these groups are virtually geometric amalgams of free groups.


1/10/2017, David Futer (Temple University)

Title: Abundant quasifuchsian surfaces in cusped hyperbolic 3-manifolds

Abstract: I will discuss a proof that every finite volume hyperbolic 3-manifold M contains an abundant collection of immersed, π1-injective surfaces. These surfaces are abundant in the sense that their lifts to the universal cover separate any pair of disjoint geodesic planes. The proof relies in a major way on the corresponding theorem of Kahn and Markovic for closed 3-manifolds. As a corollary, we recover Wise's theorem that the fundamental group of M is acts properly and cocompactly on a cube complex. This is joint work with Daryl Cooper.

1/17/2017, No Seminar

1/24/2017, Burak Ozbagci (UCLA)

Title: Fillings of unit cotangent bundles of nonorientable surfaces

Abstract: We prove that any minimal weak symplectic filling of the canonical contact structure on the unit cotangent bundle of a nonorientable closed surface other than the real projective plane is s-cobordant rel boundary to the disk cotangent bundle of the surface. If the nonorientable surface is the Klein bottle, then we show that the minimal weak symplectic filling is unique up to homeomorphism. (This is a joint work with Youlin Li.)

1/31/2017, No Seminar

2/7/2017, 5pm, Joseph Maher (CUNY College of Staten Island)

Title: Random walks on groups with coarse negative curvature.

Abstract: We will give a gentle introduction to random walks on groups satisfying various types of negative curvature conditions. A simple example is the nearest neighbour random walk on the 4-valent tree, also known as the Cayley graph of the free group on two generators. A typical random walk moves away from the origin at linear speed, and converges to one of the ends of the tree. We will discuss how to generalize this result to more general settings, such as hyperbolic groups, or acylindrical groups. This is joint work with Giulio Tiozzo.

2/14/2017, No Seminar

2/21/2017, Priyam Patel (UCSB)

Title: Algebraic and topological properties of big mapping class groups

Abstract: The mapping class group of a surface is the group of homeomorphisms of the surface up to isotopy (a natural equivalence). Mapping class groups of finite type surfaces have been extensively studied and are, for the most part, well-understood. There has been a recent surge in studying surfaces of infinite type and in this talk, we shift our focus to their mapping class groups, often called big mapping class groups. In contrast to the finite type case, there are many open questions regarding the basic algebraic and topological properties of big mapping class groups. Until now, for instance, it was unknown whether or not these groups are residually finite. We will discuss the answer to this and several other open questions after providing the necessary background on surfaces of infinite type. This work is joint with Nicholas G. Vlamis.

2/28/2017, Sergio Fenley (Florida State University)

Title: Free Seifert fibered pieces of pseudo-Anosov flows

Abstract: We prove a structure theorem for pseudo-Anosov flows restricted to Seifert fibered pieces of three manifolds. The piece is called periodic if there is a Seifert fibration so that a regular fiber is freely homotopic, up to powers, to a closed orbit of the flow. A non periodic Seifert fibered piece is called free. In this talk we consider free Seifert pieces. We show that, in a carefully defined neighborhood of the free piece, the pseudo-Anosov flow is orbitally equivalent to a hyperbolic blow up of a geodesic flow piece. A geodesic flow piece is a finite cover of the geodesic flow on a compact hyperbolic surface, usually with boundary. We introduce almost k-convergence groups, and an associated convergence theorem. We also introduce an alternative model for the geodesic flow of a hyperbolic surface that is suitable to prove these results, and we define what is a hyperbolic blow up. This is joint work with Thierry Barbot.

3/7/2017, No Seminar

3/14/2017, Stephen Trettel (UCSB)

Title: The Heisenberg Plane

Abstract: For my advancement I will talk about the Klein geometry which arises from the projective action of the Heisenberg group on the plane. This geometry is interesting in relation to other two dimensional geometries as it is the only (nontrivial) common limit of the three constant curvature geometries inside of projective geometry. However, it is also interesting in and of itself as there is a rich deformation space of Heisenberg structures on tori. I will not assume previous experience with taking limits of geometries or deformation spaces of structures, and the talk will contain a few fun mathematical animations so I hope to see you there!


4/4/2017, Nathan Williams (UCSB)

Title: Why the Fuss?

Abstract: Spherical Artin groups (like braid groups) have two surprisingly different--but equally natural--presentations. Briefly, their classical presentation arises from the chamber geometry their associated Coxeter group enjoys as a real reflection group, while their more recent dual presentation comes from interpreting the Coxeter group as a complex reflection group. In this dual presentation, certain elements of the Artin group are singled out, and these elements lead to a beautiful enumerative theory. In this talk, I will present work with C. Stump and H. Thomas that identifies a corresponding theory in the classical presentation.

4/11/2017, TBA

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4/18/2017, Catherine Pfaff (UCSB)

Title: Stable Strata of Geodesics in Outer Space

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4/25/2017, Robert Edwards (UCLA)

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5/2/2017, Giuseppe Martone (University of Southern California)

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5/9/2017, Rosalie Carlson (UCSB)

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5/16/2017, Tengren Zhang (Caltech)

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5/23/2017, Jenny Wilson (Stanford University)

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5/30/2017, Ryan Blair (Cal State Long Beach)

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6/6/2017, Erica Flapan (Pomona College)

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