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Embedded Factor Patterns for Deodhar Elements in Kazhdan-Lusztig Theory
Sara C. Billey1 and Brant C. Jones2
1Department of Mathematics, University of Washington, Box 354350, Seattle, WA 98195-4350, USA
billey@math.washington.edu
2Department of Mathematics, One Shields Avenue, University of California, Davis, CA 95616, USA
brant@math.ucdavis.edu
Annals of Combinatorics 11 (3-4) p.285-333 September, 2007
AMS Subject Classification: 20C08
Abstract:
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 simple all positive interpretation for them is known in general. Deodhar [16] 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 finiteWeyl groups. Equivalently, if (W, S) is a Coxeter system for a finite Weyl group, we classify the elements w 2W for which the Kazhdan-Lusztig basis element C'w w can be written as a monomial of C's s where s ∈S. This work generalizes results of Billey-Warrington [8] that identified the Deodhar elements in type A as 321-hexagon-avoiding permutations, and Fan-Green [18] that identified the fully-tight Coxeter groups.
Keywords: Kazhdan-Lusztig polynomials, Deodhar elements, tight element, 321-hexagon, pattern avoidance, heaps, reduced expressions, two-sided weak Bruhat order, factor

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