The Kazhdan-Lusztig polynomial of a matroid M, denoted P_M( t ), was recently defined by Elias, Proudfoot, and Wakefield. These polynomials are analogous to the classical Kazhdan-Lusztig polynomials associated with Coxeter groups. For example, in both cases there is a purely combinatorial recursive definition. Furthermore, in the classical setting, if the Coxeter group is a Weyl group then the Kazhdan-Lusztig polynomial is a Poincare polynomial for the intersection cohomology of a particular variety; in the matroid setting, if M is a realizable matroid then the Kazhdan-Lusztig polynomial is also the intersection cohomology Poincare polynomial of a variety corresponding to M. (Though there are several analogies between the two types of polynomials, the theory is quite different.) Here we compute the Kazhdan-Lusztig polynomials of several graphical matroids, including thagomizer graphs, the complete bipartite graph K_{2,n}, and (conjecturally) fan graphs. Additionally, we investigate a conjecture by the author, Proudfoot, and Young on the real-rootedness for Kazhdan-Lusztig polynomials of these matroids as well as a conjecture on the interlacing behavior of these roots. We also show that the Kazhdan-Lusztig polynomials of uniform matroids of rank n − 1 on n elements are real-rooted. This dissertation includes both previously published and unpublished co-authored material.

This mathematics thesis deals with combinatorial representation theory. Cells were introduced in a 1979 paper written by D. Kazhdan and G. Lusztig, and have intricate links with many areas of mathematics, including the representation theory of Coxeter groups, Iwahori–Hecke algebras, semisimple complex Lie algebras, reductive algebraic groups and Lie groups. One of the main problems in the theory of cells is their classification for all finite Coxeter groups. This thesis is a detailed study of cells in type Bn with respect to certain choices of parameters, and contributes to the classification by giving the first characterisation of left cells when b/a = n − 1. Other results include the introduction of a generalised version of the enhanced right descent set and exhibiting the asymptotic left cells of type Bn as left Vogan classes. Combinatorial results give rise to efficient algorithms so that cells can be determined with a computer; the methods involved in this work transfer to a new, faster way of calculating the cells with respect to the studied parameters. The appendix is a Python file containing code to make such calculations.

Thesis (Ph. D.)--University of Washington, 1999. / Vita. Includes bibliographical references (p. 79-80).

The Iwahori-Hecke algebras of Coxeter groups play a central role in the study of representations of semisimple Lie-type groups. An important tool is the combinatorial approach to representations of Iwahori-Hecke algebras introduced by Kazhdan and Lusztig in 1979. In this dissertation, I discuss a generalization of the Iwahori-Hecke algebra of the symmetric group that is instead based on the complex reflection group G(r,1,n). Using the analogues of Kazhdan and Lusztig's R-polynomials, I show that this algebra determines a partial order on G(r,1,n) that generalizes the Chevalley-Bruhat order on the symmetric group. I also consider possible analogues of Kazhdan-Lusztig polynomials.

This thesis deals with the study of Schubert varieties, which are subsets of flag varieties indexed by elements of Weyl groups. We start by defining Lascoux elements in the Hecke algebra, and showing that they coincide with the Kazhdan-Lusztig basis elements in certain cases. We then construct a resolution (Zw, π) of the Schubert variety Xw for which Rπ*(C[l(w)]) is a sheaf on Xw whose expression in the Hecke algebra is closely related to the Lascoux element. We also define two new polynomials which coincide with the intersection cohomology Poincar\'e polynomial in certain cases. In the final chapter, we discuss some interesting combinatorial results concerning Bell and Catalan numbers which arose throughout the course of this work.

Orbitales, ont démontré que les multiplicités dans une representation induitetotale sont données par les valeurs en q = 1 des polynômes de Kazhdan-Lusztig associés aux groupes symétriques. Dans ma thèse, j’ai introduit lanotion de dérivée partielle qui raffine celle de Zelevinksy et s’identifie enq = 1, à l’exponentielle formelle de la q-dérivée de Kashiwara sur l’algèbrequantique. A l’aide de cette notion et en explorant la géométrie des variétésorbitales, je construis une procédure de symétrisation des multisegments mepermettant, en particulier, de prouver une conjecture de Zelevinsky portantsur une propiété d’indépendance de l’induite parabolique totale. Je développepar ailleurs une stratégie afin de calculer les multiplicités dans une induiteparabolique générale en utilisant le produit de faisceaux pervers de Lusztig. / Ariki and Ginzburg, after the previous work of Zelevinsky on orbital varieties,proved that multiplicities in a total parabolically induced representations aregiven by the value at q = 1 of Kazhdan-Lusztig Polynomials associated to thesymmetric groups. In my thesis I introduce the notion of partial derivativewhich refines the Zelevinsky derivative and show that it can be identified withthe formal exponential of the q-derivative of Kashiwara with q=1. With thehelp of this notion, I exploit the geometry of the nilpotent orbital varietiesto construct a symmetrization process for the multi-segments, which allowsme to proove a conjecture of Zelevinsky on the property of the independenceof the total parabolic induction. On the other hand, I develop a strategyto calculate the multiplicity in a general parabolic induction by using theLusztig product of perverse sheaves.