## Seminars and Colloquia by Series

Monday, December 4, 2017 - 14:00 , Location: Skiles 006 , Soren Galatius , Stanford University , Organizer: Kirsten Wickelgren
The general linear groups GL_n(A) can be defined for any ring A, and Quillen's definition of K-theory of A takes these groups as its starting point.  If A is commutative, one may define symplectic K-theory in a very similar fashion, but starting with the symplectic groups Sp_{2n}(A), the subgroup of GL_{2n}(A) preserving a non-degenerate skew-symmetric bilinear form.  The result is a sequence of groups denoted KSp_i(A) for i = 0, 1, ....  For the ring of integers, there is an interesting action of the absolute Galois group of Q on the groups KSp_i(Z), arising from the moduli space of polarized abelian varieties.  In joint work with T. Feng and A. Venkatesh we study this action, which turns out to be an interesting extension between a trivial representation and a cyclotomic representation.
Monday, November 20, 2017 - 14:05 , Location: Skiles 006 , Kevin Kordek , Georgia Institute of Technology , Organizer: Dan Margalit
It is generally a difficult problem to compute the Betti numbers of a given finite-index subgroup of an infinite group, even if the Betti numbers of the ambient group are known. In this talk, I will describe a procedure for obtaining new lower bounds on the first Betti numbers of certain finite-index subgroups of the braid group. The focus will be on the level 4 braid group, which is the kernel of the mod 4 reduction of the integral Burau representation. This is joint work with Dan Margalit.
Monday, November 13, 2017 - 13:55 , Location: Skiles 006 , Thang Le , Georgia Tech , , Organizer: Thang Le
We discuss the growth of homonoly in finite coverings, and show that the growth of  the torsion part of the first homology of finite coverings of 3-manifolds is bounded from above by the hyperbolic volume of the manifold. The proof is based on the theory of L^2 torsion.
Monday, November 6, 2017 - 14:30 , Location: Boyd 304 , Peter Lambert-Cole and Alex Zupan , Georgia Tech and Univ. Nebraska Lincoln , Organizer: Caitlin Leverson
Peter Lambert-Cole: Mutant knots are notoriously hard to distinguish. Many, but not all, knot invariants take the same value on mutant pairs. Khovanov homology with coefficients in Z/2Z is known to be mutation-invariant, while the bigraded knot Floer homology groups can distinguish mutants such as the famous Kinoshita-Terasaka and Conway pair. However, Baldwin and Levine conjectured that delta-graded knot Floer homology, a singly-graded reduction of the full invariant, is preserved by mutation. In this talk, I will give a new proof that Khovanov homology mod 2 is mutation-invariant. The same strategy can be applied to delta-graded knot Floer homology and proves the Baldwin-Levine conjecture for mutations on a large class of tangles.  -----------------------------------------------------------------------------------------------------------------------------------------------Alex Zupan: Generally speaking, given a type of manifold decomposition, a natural problem is to determine the structure of all decompositions for a fixed manifold. In particular, it is interesting to understand the space of decompositions for the simplest objects. For example, Waldhausen's Theorem asserts that up to isotopy, the 3-sphere has a unique Heegaard splitting in every genus, and Otal proved an analogous result for classical bridge splittings of the unknot. In both cases, we say that these decompositions are "standard," since they can be viewed as generic modifications of a minimal splitting. In this talk, we examine a similar question in dimension four, proving that -- unlike the situation in dimension three -- the unknotted 2-sphere in the 4-sphere admits a non-standard bridge trisection. This is joint work with Jeffrey Meier.
Monday, October 30, 2017 - 13:55 , Location: Skiles 006 , Shea Vela-Vick , LSU , Organizer: John Etnyre
Heegaard Floer theory provides a powerful suite of tools for studying 3-manifolds and their subspaces. In 2006, Ozsvath, Szabo and Thurston defined an invariant of transverse knots which takes values in a combinatorial version of this theory for knots in the 3—sphere. In this talk, we discuss a refinement of their combinatorial invariant via branched covers and discuss some of its properties. This is joint work with Mike Wong.
Thursday, October 26, 2017 - 11:00 , Location: Skiles 006 , Nikita Selinger , University of Alabama-Birmingham , Organizer: Balazs Strenner
In a joint work with M. Yampolsky, we gave a classification of Thurston maps with parabolic orbifolds based on our previous results on characterization of canonical Thurston obstructions. The obtained results yield a  solution to the problem of algorithmically checking combinatorial equivalence of two Thurston maps.
Monday, October 23, 2017 - 13:55 , Location: Skiles 006 , Mark Hughes , BYU , Organizer: John Etnyre
The immersed Seifert genus of a knot $K$ in $S^3$ can be defined as the minimal genus of an orientable immersed surface $F$ with $\partial F = K$.  By a result of Gabai, this value is always equal to the (embedded) Seifert genus of $K$.  In this talk I will discuss the embedded and immersed cross-cap numbers of a knot, which are the non-orientable versions of these invariants.  Unlike their orientable counterparts these values do not always coincide, and can in fact differ by an arbitrarily large amount.  In further contrast to the orientable case, there are families of knots with arbitrarily high embedded 4-ball cross-cap numbers, but which are easily seen to have immersed cross-cap number 1.  After describing these examples I will discuss a classification of knots with immersed cross-cap number 1.  This is joint work with Seungwon Kim.
Monday, October 16, 2017 - 13:55 , Location: Skiles 006 , Kyle Hayden , Boston College , Organizer: John Etnyre
Every four-dimensional Stein domain has a Morse function whoseregular level sets are contact three-manifolds. This allows us to studycomplex curves in the Stein domain via their intersection with thesecontact level sets, where we can comfortably apply three-dimensional tools.We use this perspective to understand links in Stein-fillable contactmanifolds that bound complex curves in their Stein fillings.
Monday, October 9, 2017 - 13:55 , Location: Skiles 006 , None , None , Organizer: Jennifer Hom
Monday, October 2, 2017 - 15:30 , Location: Skiles 005 , Jeff Meier , UGA , Organizer: Caitlin Leverson
I'll introduce you to one of my favorite knotted objects: fibered, homotopy-ribbon disk-knots.  After giving a thorough overview of these objects, I'll discuss joint work with Kyle Larson that brings some new techniques to bear on their study.  Then, I'll present new work with Alex Zupan that introduces connections with Dehn surgery and trisections.  I'll finish by presenting a classification result for fibered, homotopy-ribbon disk-knots bounded by square knots.