The University of Michigan Combinatorics Seminar
Winter 2007
February 9, 4:10-5:00, 3866 East Hall

Embedded factor patterns for Deodhar
elements in Kazhdan-Lusztig theory

Sara Billey

University of Washington


The Kazhdan-Lusztig polynomials for finite Weyl groups arise in the geometry of Schubert varieties and representation theory. It was proved very soon after their introduction that they have nonnegative integer coefficients, but no completely combinatorial interpretation for them is known in general. Deodhar has given a framework for computing the Kazhdan-Lusztig polynomials, which generally involves recursion. We define embedded factor pattern avoidance for general Coxeter groups and use it to characterize when Deodhar's algorithm yields a simple combinatorial formula for the Kazhdan-Lusztig polynomials of finite Weyl groups. Equivalently, if (W, S) is a Coxeter system for a finite Weyl group, we classify the elements w in W for which the Kazhdan-Lusztig basis element C'_w can be written as a monomial of C'_s for s in S. This work generalizes results of Billey-Warrington which identified the Deodhar elements in type A as 321-hexagon-avoiding permutations, and Fan-Green which identified the fully-tight Coxeter groups.

This is joint work with Brant Jones.