## Seminars and Colloquia by Series

Wednesday, September 29, 2010 - 12:00 , Location: Skiles 171 , , School of Mathematics - Georgia Institute of Technology , Organizer:

Hosts: Yao Li and Ricardo Restrepo

I will consider mathematical models of decision making based on dynamics in the neighborhood of unstable equilibria and involving random perturbations due to small noise. I will report results on the vanishing noise limit for these systems, providing precise predictions about the statistics of decision making times and sequences of unstable equilibria visited by the process. Mathematically, the results are based on the analysis of random Poincare maps in the neighborhood of each equilibrium point. I will discuss applications to neuroscience and psychology along with some experimental data.
Wednesday, September 22, 2010 - 12:00 , Location: Skiles 171 , , School of Mathematics - Georgia Institute of Technology , Organizer:

Hosts: Yao Li and Ricardo Restrepo

When an object is small enough that quantum mechanics matters, many of its physical properties, such as energy levels, are determined by the eigenvalues of some linear operators. For quantum wires, waveguides, and graphs, geometry and topology show up in the operators and affect the set of eigenvalues, known as the spectrum.  It turns out that the spectrum can't be just any sequence of numbers, both because of some general theorems about the eigenvalues and because of inequalities involving the shape.  I'll discuss some of the extreme cases that test the theorems and inequalities and connect them to the shapes of the structures and to algebraic properties of the operators.To understand this lecture it would be helpful to know a little about PDEs and eigenvalues, but no knowledge of quantum mechanics will be needed.
Wednesday, September 15, 2010 - 12:00 , Location: Siles 171 , , School of Mathematics - Georgia Institute of Technology , Organizer:

Hosted by Yao Li and Ricardo Restrepo.

Deciding how to unknot a knotted piece of string (with its ends glued together) is not only a difficult problem in the real world, it is also a difficult and long studied problem in mathematics. (There are several notions of what one might mean by "unknotting" and I will leave the exact meaning a bit vague in this abstract.) In the past mathematicians have used a vast array of techniques --- from geometry to algebra, and even PDEs --- to study this question. I will discuss this question and (partially) recast it in terms of 4 dimensional topology. This new perspective will allow us to use a powerful new knot invariant called Khovanov Homology to study the problem. I will give an overview of Khovanov Homology and indicate how to study our unknotting question using it.
Wednesday, September 8, 2010 - 12:00 , Location: Skiles 171 (NOTICE THE CHANGE OF ROOM) , , School of Mathematics - Georgia Institute of Technology , Organizer:

Hosted by: Yao Li and Ricardo Restrepo

Combinatorial mathematics exhibits a number of elegant, simply stated problems that turn out to be surprisingly challenging. In this talk, I report on a problem of this type on which I have been working with Noah Streib, Stephen Young and Ruidong Wang from Georgia Tech, as well as Piotr Micek, Bartek Walczak and Tomek Krawczyk, all computer scientists from Poland. Given positive integers  $k$ and $w$, what is the largest integer   $t = f(k,w)$  for which there exists a family $\mathcal{F}$ of $t$ vectors in $N^{w}$ so that: \begin{enumerate} \item  Any two vectors in the family $\mathcal{F}$ are incomparable in the product ordering; and \item  There do not exist two vectors $A$ and $B$ in the family for which there are distinct  $i$ and $j$  so that  $a_i\ge k +b_i$  and $b_j \ge k + a_j$. \end{enumerate}  The Polish group posed the problem to us at the SIAM Discrete Mathematics held in Austin, Texas, this summer.  They were able to establish the following bounds: $k^{w-1} \le t \le k^w$ We were able to show that the lower bound is essentially correct by showing that there is a constant  $c_w$   so that  $t \l c_w k^{w-1}$. But recent work suggests that the lower bound might actually be tight.
Wednesday, September 1, 2010 - 12:00 , Location: Skiles 114 , , School of Mathematics - Georgia Tech , Organizer:
Orthogonal Polynomials play a key role in analysis of random matrices. We discuss universality limits in the so-called unitary case, showing how the universality limit reduces to an asymptotic involving reproducing kernels associated with orthogonal polynomials. As a consequence, we show that universality holds in measure for any compactly supported measure.
Tuesday, April 27, 2010 - 12:00 , Location: Skiles 255 , Igor Belegradek , Professor, School of Mathematics , Organizer:

Hosted by: Huy Huynh and Yao Li

A starting point of geometric group theory is thinking of a group as a geometric object, by giving it a metric induced from the Cayley graph of the group. Gromov initiated a program of studying groups up to quasi-isometries, which are bilipschitz maps up to bounded additive error". Quasi-isometries ignore local structure and preserve asymptotic properties of a metric space. In the talk I will give a sample of results, examples, and open questions in this area.
Tuesday, April 20, 2010 - 12:00 , Location: Skiles 255 , Douglas Ulmer , Professor and Chair, School of Mathematics , Organizer:

Hosted by: Huy Huynh and Yao Li

Elliptic curves are solution sets of cubic polynomials in two variables.  I'll explain a bit of where they came from (computing the arc length of an ellipse, hence the name), their remarkable group structure, and some of the many roles they play in mathematics and applications, from mechanics to algebraic geometry  to cryptography.
Tuesday, April 13, 2010 - 12:00 , Location: Skiles 255 , Jeff Geronimo , Professor, School of Mathematics , Organizer:

Hosted by: Huy Huynh and Yao Li

A useful parametrization of the one variable trigonometric moment problem is given in terms of polynomials orthogonal on the unit circle. A description of this parameterization will be given as well as some of its uses. We will then describe a possible two variable extension.
Tuesday, April 6, 2010 - 12:00 , Location: Skiles 255 , Anton Leykin , School of Math, Georgia Tech , Organizer:

Hosted by: Huy Huynh and Yao Li

One of the basic problems arising in many pure and applied areas of mathematics is to solve a system of polynomial equations. Numerical Algebraic Geometry starts with addressing this fundamental problem and develops machinery to describe higher-dimensional solution sets (varieties) with approximate data. I will introduce numerical polynomial homotopy continuation, a technique that is radically different from the classical symbolic approaches as it is powered by (inexact) numerical methods.
Tuesday, March 30, 2010 - 12:00 , Location: Skiles 255 , Huy and Yao , School of Math , Organizer:

Hosted by: Huy and Yao

Research Horizons features Lunch Fun Break! The purpose is to create an opportunity for all graduate students, new and experienced, domestic and international, to meet, eat and have fun.AGENDA: ***"Suggestion box" for graduate students will be displayed in Faculty Lounge Skiles 236.***  Propective students' visit on Friday, April 2. *** Game: "Can you comunicate in silience?" *** PIZZAs, soft DRINKs, relax and have fun. ***