### DIMACS - RUTGERS EXPERIMENTAL MATHEMATICS SEMINAR

Sponsored by the Rutgers University Department of Mathematics and the

Center for Discrete Mathematics and Theoretical Computer Science (DIMACS)

**Co-organizers:**
**Matthew Russell**, Rutgers University, russell2 {at} math [dot] rutgers [dot] edu)
** Doron Zeilberger**, Rutgers University, zeilberg {at} math [dot] rutgers [dot] edu

Title: q-Combinatorics: A new view

Speaker: **Margaret Readdy**, University of Kentucky and Princeton University

Date: Thursday, March 12, 2015 5:00pm

Location: Hill Center, Room 705, Rutgers University, Busch Campus, Piscataway, NJ

Abstract:

A q-analogue is a method to enumerate a set of objects by keeping track of one or more of its mathematical properties. After setting q=1, one returns to the naive enumeration.

After reviewing some classical q-analogues, we will discuss the new idea of a negative q-analogue. This concept is motivated by Fu, Reiner, Stanton and Thiem's recent work on the negative q-binomial. We show the classical q-Stirling numbers of the second kind can be expressed as a pair of statistics on a subset of restricted growth words. The resulting expressions are polynomials in q and 1+q. We extend this enumerative result via a decomposition of the Stirling poset of the first kind, as well as a homological version of Stembridge's q = -1 phenomenon.

We then describe a parallel enumerative, poset theoretic and homological study for the q-Stirling numbers of the first kind. Time permitting, we give a bijective combinatorial argument a la Viennot showing the (q,t)-Stirling numbers of the first and second kind are orthogonal.

This is joint work with Yue Cai.

See: http://www.math.rutgers.edu/~russell2/expmath/