Seminars and Colloquia by Series

Fractional powers of Dehn twists about nonseparating curves

Series
Geometry Topology Seminar
Time
Monday, May 14, 2012 - 14:05 for 1 hour (actually 50 minutes)
Location
Skiles 005
Speaker
Kashyap Rajeevsarathy – IISER Bhopal
Let S_g be a closed orientable surface of genus g > 1 and C a simple closed nonseparating curve in S_g. Let t_C denote a left handed Dehn twist about C. A fractional power of t_C of exponent L/n is a h in Mod(S_g) such that h^n = t_C^L. Unlike a root of a t_C, a fractional power h can exchange the sides of C. We will derive necessary and sufficient conditions for the existence of both side-exchanging and side-preserving fractional powers. We will give some applications of the main result in both cases. Finally, we give a complete classification of a certain class of side-preserving and side-exchanging fractional powers on S_5.

Surface bundles, Lefschetz fibrations, and their (multi)sections

Series
Geometry Topology Seminar
Time
Monday, May 7, 2012 - 14:00 for 1 hour (actually 50 minutes)
Location
Skiles 005
Speaker
Inanc Baykur – Max Planck
Surface bundles and Lefschetz fibrations over surfaces constitute a rich source of examples of smooth, symplectic, and complex manifolds. Their sections and multisections carry interesting information on the smooth structure of the underlying four-manifold. In this talk we will discuss several problems and results on surface bundles, Lefschetz fibrations, and their (multi)sections, which we will tackle, for the most part, using various mapping class groups of surfaces. Conversely, we will use geometric arguments to obtain some structural results for mapping class groups.

Grid Movie Moves and Combinatorial Knot Floer Homology

Series
Geometry Topology Seminar
Time
Monday, April 16, 2012 - 14:00 for 1 hour (actually 50 minutes)
Location
Skiles 005
Speaker
Matt Graham – Brandeis University
Recently, Sarkar showed that a smooth marked cobordism between two knots K_1 , K_2 induces a map between the knot Floer homology groups of the two knots HFK(K_1 ), HFK(K_2 ). It has been conjectured that this map is well defined (with respect to smooth marked cobordisms). After outlining what needs to be shown to prove this conjecture, I will present my current progress towards showing this result for the combinatorial version of HFK. Specifically, I will present a generalization of Carter and Saito's movie move theorem to grid diagrams, give a very brief introduction to combinatorial knot Floer homology and then present a couple of the required chain homotopies needed for the proof of the conjecture.

Irreducible factors of modular representations of mapping class groups arising in Integral TQFT

Series
Geometry Topology Seminar
Time
Monday, April 9, 2012 - 14:05 for 1 hour (actually 50 minutes)
Location
Skiles 005
Speaker
Pat Gilmer – Louisiana State University
We find decomposition series of length at most two for modular representations in characteristic p of mapping class groups of surfaces induced by an integral version of the Witten-Reshetikhin-Turaev SO(3)-TQFT at the p-th root of unity. This joint work with Gregor Masbaum.

Berge duals and universally tight contact structures

Series
Geometry Topology Seminar
Time
Monday, April 2, 2012 - 14:00 for 1 hour (actually 50 minutes)
Location
Skiles 005
Speaker
Chris Cornwell – Duke University
Berge has a construction that produces knots in S^3 that admit a lens space surgery. Conjecturally, his construction produces all such knots. This talk will consider knots that have such a surgery, and some of their contact geometric properties. In particular, knots in S^3 with a lens space surgery are fibered, and they all support the tight contact structure on S^3. From recent work of Hedden and Plamenevskaya, we also know that the dual to a lens space surgery on such a knot supports a tight contact structure on the resulting lens space. We consider the knots that are dual to Berge's knots, and we investigate whether the tight contact structure they support is a universally tight structure. Our results indicate a relationship between supporting this universally tight structure and being dual to a torus knot.

Curve complex translation lengths

Series
Geometry Topology Seminar
Time
Monday, March 26, 2012 - 14:05 for 1 hour (actually 50 minutes)
Location
Skiles 005
Speaker
Vaibhav Gadre – Harvard University
The curve complex C(S) of a closed orientable surface S of genusg is an infinite graph with vertices isotopy classes of essential simpleclosed curves on S with two vertices adjacent by an edge if the curves canbe isotoped to be disjoint. By a celebrated theorem of Masur-Minsky, thecurve complex is Gromov hyperbolic. Moreover, a pseudo-Anosov map f of Sacts on C(S) as a hyperbolic isometry with "north-south" dynamics and aninvariant quasi-axis. One can define an asymptotic translation length for fon C(S). In joint work with Chia-yen Tsai, we prove bounds on the minimalpseudo-Anosov asymptotic translation lengths on C(S) . We shall alsooutline related interesting results and questions.

Asymptotic Geometry of Teichmuller Space and Divergence

Series
Geometry Topology Seminar
Time
Monday, March 12, 2012 - 14:05 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Harold Sultan – Columbia University
I will talk about the asymptotic geometry of Teichmuller space equipped with the Weil-Petersson metric. In particular, I will give a criterion for determining when two points in the asymptotic cone of Teichmuller space can be separated by a point; motivated by a similar characterization in mapping class groups by Behrstock-Kleiner-Minsky-Mosher and in right angled Artin groups by Behrstock-Charney. As a corollary, I will explain a new way to uniquely characterize the Teichmuller space of the genus two once punctured surface amongst all Teichmuller space in that it has a divergence function which is superquadratic yet subexponential.

On triangulating a square

Series
Geometry Topology Seminar
Time
Monday, February 27, 2012 - 14:05 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Aaron Abrams – Emory University
I will discuss the following geometric problem. If you are given an abstract 2-dimensional simplicial complex that is homeomorphic to a disk, and you want to (piecewise linearly) embed the complex in the plane so that the boundary is a geometric square, then what are the possibilities for the areas of the triangles? It turns out that for any such simplicial complex there is a polynomial relation that must be satisfied by the areas. I will report on joint work with Jamie Pommersheim in which we attempt to understand various features of this polynomial, such as the degree. One thing we do not know, for instance, if this degree is expressible in terms of other known integer invariants of the simplicial complex (or of the underlying planar graph).

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