Seminars and Colloquia by Series

Thursday, June 23, 2016 - 14:00 , Location: Skiles 005 , Yajing Liu , UCLA , Organizer: John Etnyre
Existence of tight contact structures is a fundamental question of contact topology. Etnyre and Honda first gave the example which doesn't admit any tight structure. The existence of fillable tight structures is also a subtle question. Here we give some new examples of hyperbolic 3-manifolds which do not admit any fillable structures.
Monday, June 6, 2016 - 14:05 , Location: Skiles 114 , Christian Zickert , University of Maryland , , Organizer: Stavros Garoufalidis
We discuss the higher Teichmuller space A_{G,S} defined by Fockand Goncharov. This space is defined for a punctured surface S withnegative Euler characteristic, and a semisimple, simply connected Lie groupG. There is a birational atlas on A_{G,S} with a chart for each idealtriangulation of S. Fock and Goncharov showed that the transition functionsare positive, i.e. subtraction-free rational functions. We will show thatwhen G has rank 2, the transition functions are given by explicit quivermutations.
Tuesday, May 31, 2016 - 14:00 , Location: Skiles 006 , Caitlin Leverson , Duke University , Organizer: John Etnyre
Given a plane field $dz-xdy$ in $\mathbb{R}^3$. A Legendrian knot is a knot which at every point is tangent to the plane at that point. One can similarly define a Legendrian knot in any contact 3-manifold (manifold with a plane field satisfying some conditions). In this talk, we will explore Legendrian knots in $\mathbb{R}^3$, $J^1(S^1)$, and $\#^k(S^1\times S^2)$ as well as a few Legendrian knot invariants. We will also look at the relationships between a few of these knot invariants. No knowledge of Legendrian knots will be assumed though some knowledge of basic knot theory would be useful.
Monday, May 16, 2016 - 14:00 , Location: Skiles 006 , Jennifer Hom , Georgia Tech , Organizer: John Etnyre
Auckly gave two examples of irreducible integer homology spheres (one toroidal and one hyperbolic) which are not surgery on a knot in the three-sphere. Using an obstruction coming from Heegaard Floer homology, we will provide infinitely many hyperbolic examples, as well as infinitely many examples with arbitrary JSJ decomposition. This is joint work with Lidman.
Monday, April 25, 2016 - 14:00 , Location: Skiles 006 , Nicholas J. Kuhn , University of Virginia , Organizer: Kirsten Wickelgren
In a 1958 paper, Milnor observed that then new work by Bott allowed him to show that the n sphere admits a  vector bundle with non-trivial top Stiefel-Whitney class precisely when n=1,2,4, 8.  This can be interpreted  as a calculation of the mod 2 Hurewicz map for the classifying space BO, which has the structure of an  infinite loopspace. I have been studying Hurewicz maps for infinite loopspaces by showing how a filtration of the homotopy  groups coming from stable homotopy theory (the Adams filtration) interacts with a filtration of the homology groups coming from infinite loopspace theory. There are some clean and tidy consequences that lead to a new proof of Milnor's theorem, and other applications.
Tuesday, April 19, 2016 - 13:00 , Location: Skiles 006 , David Gay , University of Georgia , Organizer: John Etnyre

Please note different day and time for the seminar

In honor of John Stallings' great paper, "How not to prove the Poincare conjecture", I will show how to reduce the smooth 4-dimensional Poincare conjecture to a (presumably incredibly difficult) statement in group theory. This is joint work with Aaron Abrams and Rob Kirby. We use trisections where Stallings used Heegaard splittings.
Monday, April 11, 2016 - 14:00 , Location: Skiles 006 , Jonathan Beardsley , Johns Hopkins University , Organizer: Kirsten Wickelgren
Given an action by a loop space on a structured ring spectrum we describe how to produce its associated quotient ring spectrum.  We then describe how this structure may be leveraged to produce intermediate Hopf-Galois extensions of ring spectra, analogous to the way one produces intermediate Galois extensions from normal subgroups of a Galois group. We will give many examples of this structure in classical cobordism spectra and in particular describe an entirely new construction of the complex cobordism spectrum which bears a striking resemblance to Lazard's original construction of the Lazard ring by iterated extensions.
Friday, April 1, 2016 - 14:05 , Location: Skiles 006 , William Menasco , U Buffalo , Organizer: Dan Margalit
In Watchareepan Atiponrat's thesis the properties of decomposable exact Lagrangian codordisms betweenLegendrian links in R^3 with the standard contact structure were studied.  In particular, for any decomposableexact Lagrangian filling L of a Legendrian link K, one may obtain a normal ruling of K associated with L.Atiponrat's main result is that the associated normal rulings must have an even number of clasps.  As a result, there exists a Legendrian (4,-(2n +5))-torus knot, for each n >= 0, which does not have a decomposable exact Lagrangian filling because it has only 1 normal ruling and this normal rolling has odd number of clasps.
Wednesday, March 30, 2016 - 17:05 , Location: Skiles 006 , Henry Segerman , University of Oklahoma , , Organizer: Stavros Garoufalidis
When visualising topological objects via 3D printing, we need athree-dimensional geometric representation of the object. There areapproximately three broad strategies for doing this: "Manual" - usingwhatever design software is available to build the object by hand;"Parametric/Implicit" - generating the desired geometry using aparametrisation or implicit description of the object; and "Iterative" -numerically solving an optimisation problem.The manual strategy is unlikely to produce good results unless the subjectis very simple. In general, if there is a reasonably canonical geometricstructure on the topological object, then we hope to be able to produce aparametrisation of it. However, in many cases this seems to be impossibleand some form of iterative method is the best we can do. Within theparametric setting, there are still better and worse ways to proceed. Forexample, a geometric representation should demonstrate as many of thesymmetries of the object as possible. There are similar issues in makingthree-dimensional representations of higher dimensional objects. I willdiscuss these matters with many examples, including visualisation offour-dimensional polytopes (using orthogonal versus stereographicprojection) and Seifert surfaces (comparing my work with Saul Schleimerwith Jack van Wijk's iterative techniques).I will also describe some computational problems that have come up in my 3D printed work, including the design of 3D printed mobiles (joint work withMarco Mahler), "Triple gear" and a visualisation of the Klein Quartic(joint work with Saul Schleimer), and hinged surfaces with negativecurvature (joint work with Geoffrey Irving).
Monday, March 14, 2016 - 14:05 , Location: Skiles 006 , Jing Tao , U Oklahoma , Organizer: Dan Margalit
The commutator length of an element g in the commutator subgroup [G,G] of agroup G is the smallest k such that g is the product of k commutators. WhenG is the fundamental group of a topological space, then the commutatorlength of g is the smallest genus of a surface bounding a homologicallytrivial loop that represents g. Commutator lengths are notoriouslydifficult to compute in practice. Therefore, one can ask for asymptotics.This leads to the notion of stable commutator length (scl) which is thespeed of growth of the commutator length of powers of g. It is known thatfor n > 2, SL(n,Z) is uniformly perfect; that is, every element is aproduct of a bounded number of commutators, and hence scl is 0 on allelements. In contrast, most elements in SL(2,Z) have positive scl. This isrelated to the fact that SL(2,Z) acts naturally on a tree (its Bass-Serretree) and hence has lots of nontrivial quasimorphisms.In this talk, I will discuss a result on the stable commutator lengths inright-angled Artin groups. This is a broad family of groups that includesfree and free abelian groups. These groups are appealing to work withbecause of their geometry; in particular, each right-angled Artin groupadmits a natural action on a CAT(0) cube complex. Our main result is anexplicit uniform lower bound for scl of any nontrivial element in anyright-angled Artin group. This work is joint with Talia Fernos and MaxForester.