Seminars and Colloquia by Series

Total diameter and area of closed submanifolds

Series
Geometry Topology Seminar
Time
Monday, December 2, 2013 - 14:00 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Mohammad Ghomi – Georgia Tech –
The total diameter of a closed planar curve C is the integral of its antipodal chord lengths. We show that this quantity is bounded below by twice the area of C. Furthermore, when C is convex or centrally symmetric, the lower bound is twice as large. Both inequalities are sharp and the equality holds in the convex case only when C is a circle. We also generalize these results to m dimensional submanifolds of R^n, where the "area" will be defined in terms of the mod 2 winding numbers of the submanifold about the n-m-1 dimensional affine subspaces of R^n.

Vassiliev Invariants of Virtual Legendrian Knots

Series
Geometry Topology Seminar
Time
Monday, November 25, 2013 - 14:00 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Patricia Cahn – University of Pennsylvania
We introduce a theory of virtual Legendrian knots. A virtual Legendrian knot is a cooriented wavefront on an oriented surface up to Legendrian isotopy of its lift to the unit cotangent bundle and stabilization and destablization of the surface away from the wavefront. We show that the groups of Vassiliev invariants of virtual Legendrian knots and of virtual framed knots are isomorphic. In particular, Vassiliev invariants cannot be used to distinguish virtual Legendrian knots that are isotopic as virtual framed knots and have equal virtual Maslov numbers. This is joint work with Asa Levi.

The Kawamuro Cone and the Jones Conjecture

Series
Geometry Topology Seminar
Time
Friday, November 22, 2013 - 14:05 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Bill Menasco – U at Buffalo
We show that after stabilizations of opposite parity and braid isotopy, any twobraids in the same topological link type cobound embedded annuli. We use this to prove thegeneralized Jones conjecture relating the braid index and algebraic length of closed braidswithin a link type, following a reformulation of the problem by Kawamuro. This is joint workwith Doug Lafountain.

Fixed points of unitary decomposition complexes

Series
Geometry Topology Seminar
Time
Monday, November 18, 2013 - 14:00 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Vesna Stojanoska – MIT –
For a fixed integer n, consider the nerve L_n of the topological poset of orthogonal decompositions of complex n-space into proper orthogonal subspaces. The space L_n has an action by the unitary group U(n), and we study the fixed points for subgroups of U(n). Given a prime p, we determine the relatively small class of p-toral subgroups of U(n) which have potentially non-empty fixed points. Note that p-toral groups are a Lie analogue of finite p-groups, thus if we are interested in the U(n)-space L_n at a fixed prime p, only the p-toral subgroups of U(n) play a significant role. The space L_n is strongly related to the K-theory analogues of the symmetric powers of spheres and the Weiss tower for the functor that assigns to a vector space V the classifying space BU(V). Our results are a step toward a K-theory analogue of the Whitehead conjecture as part of the program of Arone-Dwyer-Lesh. This is joint work with J.Bergner, R.Joachimi, K.Lesh, K.Wickelgren.

Colored Jones polynomials and double affine Hecke algebras

Series
Geometry Topology Seminar
Time
Monday, November 11, 2013 - 14:00 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Peter Samuelson – University of Toronto
Frohman and Gelca showed that the Kauffman bracket skein module of the thickened torus is the Z_2 invariant subalgebra A'_q of the quantum torus A_q. This shows that the Kauffman bracket skein module of a knot complement is a module over A'_q. We discuss a conjecture that this module is naturally a module over the double affine Hecke algebra H, which is a 3-parameter family of algebras which specializes to A'_q. We give some evidence for this conjecture and then discuss some corollaries. If time permits we will also discuss a related topological construction of a 2-parameter family of H-modules associated to a knot in S^3. (All results in this talk are joint with Yuri Berest.)

All finite groups are involved in the mapping class group

Series
Geometry Topology Seminar
Time
Friday, November 8, 2013 - 14:05 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
G. Masbaum – Institut de Mathématiques de Jussieu
Let g be a positive integer and let Gamma_g be the mapping class group of the genus g closed orientable surface. We show that every finite group is involved in Gamma_g. (Here a group G is said to be involved in a group Gamma if G is isomorphic to a quotient of a subgroup of Gamma of finite index.) This answers a question asked by U. Hamenstadt. The proof uses quantum representations of mapping class groups. (Joint work with A. Reid.)

The structure of high distance Heegaard splittings

Series
Geometry Topology Seminar
Time
Monday, November 4, 2013 - 14:00 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Jesse Johnson – Oklahoma State University
The notion of distance for a Heegaard splitting of athree-dimensional manifold $M$, introduced by John Hempel, has provedto be a very powerful tool for understanding the geometry and topologyof $M$. I will describe how distance, and a slight generalizationknown as subsurface projection distance, can be used to explore theconnection between geometry and topology at the center of the moderntheory hyperbolic three-manifolds.In particular, Schalremann-Tomova showed that if a Heegaard splittingfor $M$ has high distance then it will be the only irreducibleHeegaard splitting of $M$ with genus less than a certain bound. I willexplain this result in terms of both a geometric proof and atopological proof. Then, using the notion of subsurface distance, Iwill describe a construction of a manifold with multiple distinctlow-distance Heegaard splittings of the same (small) genus, and amanifold with both a high distance, low-genus Heegaard splitting and adistinct, irreducible high-genus, low-distance Heegaard splitting.

Tight small Seifert fibered manifolds

Series
Geometry Topology Seminar
Time
Monday, October 28, 2013 - 14:00 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Bulent Tosun – University of Virginia
Contact geometry in three dimensions is a land of two disjoint classes ofcontact structures; overtwisted vs. tight. The former ones are flexible,means their geometry is determined by algebraic topology of underlying twoplane fields. In particular their existence and classification areunderstood completely. Tight contact structure, on the other hand, arerigid. The existence problem of a tight contact structure on a fixed threemanifold is hard and still widely open. The classification problem is evenharder. In this talk, we will focus on the classification of tight contactstructures on Seifert fibered manifolds on which the existence problem oftight contact structures was settled recently by Lisca and Stipsicz.

Essential spunnormal surfaces via tropical geometry

Series
Geometry Topology Seminar
Time
Monday, October 7, 2013 - 14:05 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Andrew Brasile – University of Illinois at Chicago –
In a paper published in 2012, Nathan Dunfield and StavrosGaroufalidis gave simple, sufficient conditions for a spunnormal surface tobe essential in a compact, orientable 3-manifold with torus boundary. Thistalk will discuss a generalization of this result which utilizes a theoremfrom tropical geometry.

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