# Seminars and Colloquia

## Topology et al. Seminar

Apr 18

#### Topology Seminar

04:20 pm

Exley Science Center Tower ESC 638

Melissa Zhang, Boston College Symmetries in topological spaces and homology-type invariants Abstract:Topologists often encounter spaces with interesting symmetries. By analyzing the symmetries of an object through the regularities of its algebraic invariants, we are able to learn more about the object and its relationship with smaller, less complex objects. For example, by using the right tools, we can easily see that for a topological space Xequipped with a cyclic action, the rank of the singular homology of Xis at least that of the fixed point set. In low-dimensional topology, knots and links are ubiquitous and far-reaching in their associations. One particular interesting algebraic invariant of links is Khovanov homology, a combinatorial homology theory whose graded Euler characteristic is the Jones polynomial. In this talk, we consider links exhibiting 2-fold symmetry and prove a rank inequality for a variant of Khovanov homology.

Feb 21

#### Topology Seminar

04:20 pm

Exley Science Center Tower ESC 638

Kyle Hayden, Boston College From algebraic curves to ribbon disks and back Abstract: There is a rich, symbiotic relationship between knot theory and the study of algebraic curves in complex spaces. In particular, intersecting an algebraic curve in C^2 with a three-sphere of constant radius yields a knot or link that contains useful information about the algebraic curve. We'll look at a thirty-year-old conjecture about the relationship between different knots associated to the same algebraic curve. Using a simple construction, we'll show that the conjecture is false and that counterexamples are quite common.

Nov 29

#### Topology Seminar

04:20 pm

Exley Science Center Tower ESC 638

Patrick Devlin, Yale University Topological Methods in Combinatorics Abstract: It is sometimes remarked that combinatorics is not the study of structures or theorems, so much as the study of techniques. Several techniques are well known and evidently quite fruitful (e.g., linear programming, the probabilistic method, Fourier analysis, entropy, et cetera). But another tool that should be in every discrete mathematician's backpocket is topology. In this talk, we will discuss some clever applications of topology to combinatorics providing quick primers on any relevant topological (and combinatorial) concepts along the way. We will focus on work related to graph colorings and boolean function complexity and introduce several open problems and conjectures in the area.

Oct 25

#### Topology Seminar

04:20 pm

Exley Science Center Tower ESC 638

Speaker: Arie Levit, Yale Title: Local rigidity of uniform lattices Abstract: A lattice is topologically locally rigid (t.l.r) if small deformations of it are isomorphic lattices. Uniform lattices in Lie groups were shown to be t.l.r by Weil [60']. We show that uniform lattices in any compactly generated topological group are t.l.r. A lattice is locally rigid (l.r) if small deformations arise from conjugation. It is a classical fact due to Weil [62'] that lattices in semi-simple Lie groups are l.r. Relying on our t.l.r results and on recent work by Caprace-Monod we prove l.r for uniform lattices in the isometry groups of certain CAT(0) spaces, with the exception of SL_2(R), which occurs already in the classical case. In the talk I will explain the above notions and results, and present some geometric ideas from the proofs. This is a joint work with Tsachik Gelander.

Oct 11

#### Topology Seminar

04:20 pm

Exley Science Center Tower ESC 638

Speaker: Noelle Sawyer, Wes Title: The specification property and closed orbit measures Abstract: Given a compact metric space X, and a homeomorphism T on X, the dynamical system (X,T) has the specification property if we can approximate distinct orbit segments with the orbit of a periodic point. I will present a result by Sigmund showing that if (X,T) has the specification property, any T-invariant measure on X can be approximated by a closed orbit measure.

Oct 4

#### Topology Seminar

04:20 pm

Exley Science Center Tower ESC 638

Speaker: Shelly Harvey, Rice University Title: Braids, gropes, Whitney towers, and solvability of links Abstract: The n-solvable filtrations of the knot/link concordance groups were defined as a way of studying the structure of the groups and in particular, the subgroup of algebraically slice knots/links. While the knot concordance group C^1 is known to be an abelian group, when m is at least 2, the link concordance group C^m of m-component (string) links is known to be non-abelian. In particular, it is well known that the pure braid group with m strings is a subgroup of C^m and hence when m is at least 3, this shows that C^m contains a non-abelian free subgroup. We study the relationship between the derived subgroups of the the pure braid group, n-solvable filtration of C^m, links bounding symmetric Whitney towers, and links bounding gropes. This is joint with with Jung Hwan Park and Arunima Ray.

Sep 27

#### Topology Seminar

04:20 pm

Exley Science Center Tower ESC 638

Speaker: Samuel Lin, Dartmouth Title: Curvature Free Rigidity of Higher Rank Three- manifolds Abstract: Fixing K=-1, 0, or 1, a Riemannian manifold (M, g) is said to have higher hyperbolic, spherical, or Euclidean rank if every geodesic in M admits a normal parallel field making curvature K with the geodesic. Rank rigidity results, which usually involve a priori sectional curvature bounds, characterize locally symmetric spaces in terms of these geometric notions of rank. After giving a short survey of historical results, Ill discuss how rank rigidity holds in dimension three without a priori sectional curvature bounds.

Nov 30

#### Topology Seminar

04:20 pm

Exley Science Center Tower ESC 121

Boris Gutkin, University Duisberg-Essen: Pairings between periodic orbits in hyperbolic coupled map lattices. Abstract: Upon quantization, hyperbolic Hamiltonian systems generically exhibit universal spectral properties effectively described by Random Matrix Theory. Semiclassically this remarkable phenomenon can be attributed to the existence of pairs of classical periodic orbits with small action differences. So far, however, the scope of this theory has, by and large, been restricted to small-dimensional systems. I will discuss an extension of this program to hyperbolic coupled map lattices with a very large number of sites. The crucial ingredient is a two-dimensional symbolic dynamics which allows an effective representation of periodic orbits and their pairings. I will illustrate the theory with a specific model of coupled cat maps, where such a symbolic dynamics can be constructed explicitly. The core of the talk is based on the joint work with V. Osipov: Nonlinearity 29, 325 (2016) and a work in progress with P. Cvitanovic, R. Jafari, L. Han, A. Saremi.

Sep 28

#### Topology Seminar

04:20 pm

Exley Science Center Tower ESC 638

Dave Constantine, Wes: " Hausdorff dimension and the CAT(K) condition for surfaces" Abstract: A geodesic metric space satisfies the CAT(K) condition if its geodesic triangles are all `thinner' than triangles with the same side lengths in the model space of constant Riemannian curvature K. This condition allows one to extend many arguments relying on an upper curvature bound from Riemannian geometry to the metric space setting. How `strange' can a metric be while still satisfying the CAT(K) property? One way to measure this is with the difference between the topological dimension of the space and its Hausdorff dimension with respect to the metric. In this talk I'll show that, at least for surfaces, a CAT(K) metric is tame in the sense that it yields Hausdorff dimension 2. I'll also provide some motivation for this question by showing how results like this allow one to extend volume entropy rigidity statements to the CAT(-1) setting.