The University of Michigan Combinatorics Seminar


Abstract 

The KazhdanLusztig 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 KazhdanLusztig 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 KazhdanLusztig 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 KazhdanLusztig basis element C'_w can be written as a monomial of C'_s for s in S. This work generalizes results of BilleyWarrington which identified the Deodhar elements in type A as 321hexagonavoiding permutations, and FanGreen which identified the fullytight Coxeter groups. This is joint work with Brant Jones. 