Georgia Institute of Technology College of Sciences

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.

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.

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.

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. ***