The cd-index of Bruhat Intervals

We study flag enumeration in intervals in the Bruhat order on a Coxeter group by means of a structural recursion on intervals in the Bruhat order. The recursion gives the isomorphism type of a Bruhat interval in terms of smaller intervals, using basic geometric operations which preserve PL sphericity and have a simple effect on the cd-index. This leads to a new proof that Bruhat intervals are PL spheres as well a recursive formula for the cd-index of a Bruhat interval. This recursive formula is used to prove that the cd-indices of Bruhat intervals span the space of cd-polynomials. The structural recursion leads to a conjecture that Bruhat spheres are “smaller” than polytopes. More precisely, we conjecture that if one fixes the lengths of x and y, then the cd-index of a certain dual stacked polytope is a coefficientwise upper bound on the cd-indices of Bruhat intervals [x, y]. We show that this upper bound would be tight by constructing Bruhat intervals which are the face lattices of these dual stacked polytopes. As a weakening of a special case of the conjecture, we show that the flag h-vectors of lower Bruhat intervals are bounded above by the flag h-vectors of Boolean algebras (i. e. simplices). A graded poset is Eulerian if in every non-trivial interval, the number of elements of odd rank equals the number of elements of even rank. Face lattices of convex polytopes are in particular Eulerian and the study of flag enumeration in Eulerian posets has its origins in the face-enumeration problem for polytopes. All flag-enumerative information in an Eulerian poset P can be encapsulated in a non-commutative generating function ΦP called the cd-index. The cd-indices of polytopes have received much attention, for example in [1, 2, 8, 11, 18]. ∗The author was partially supported by the Thomas H. Shevlin Fellowship from the University of Minnesota Graduate School and by NSF grant DMS-9877047. This article consists largely of material from the author’s doctoral thesis [14]. the electronic journal of combinatorics 11 (2004), #R74 1 A Coxeter group is a group generated by involutions, subject to certain relations. Important examples include finite reflection groups and Weyl groups. The Bruhat order on a Coxeter group is a partial order which has important connections to the combinatorics and representation theory of Coxeter groups, and by extension Lie algebras and groups. Intervals in Bruhat order comprise another important class of Eulerian posets. However, flag enumeration for intervals in the Bruhat order on a Coxeter group has previously received little attention. The goal of the present work is to initiate the study of the cd-index of Bruhat intervals. The basic tool in our study is a fundamental structural recursion (Theorem 5.5) on intervals in the Bruhat order on Coxeter groups. This recursion, although developed independently, has some resemblance to work by du Cloux [6] and by Dyer [7]. The recursion gives the isomorphism type of a Bruhat interval in terms of smaller intervals, using some basic geometric operations, namely the operations of pyramid, vertex shaving and a “zipping” operation. The result is a new inductive proof of the fact [3] that Bruhat intervals are PL spheres (Corollary 5.6) as well as recursions for the cd-index of Bruhat intervals (Theorem 6.1). The recursive formulas lead to a proof that the cd-indices of Bruhat intervals span the space of cd-polynomials (Theorem 6.2), and motivate a conjecture on the upper bound for the cd-indices of Bruhat intervals (Conjecture 7.3). Let [u, v] be an interval in the Bruhat order such that the rank of u is k and the rank of v is d + k + 1. We conjecture that the coefficients of Φ[u,v] are bounded above by the coefficients of the cd-index of a dual stacked polytope of dimension d with d + k + 1 facets. The dual stacked polytopes are the polar duals of the stacked polytopes of [12]. This upper bound would be sharp because the structural recursion can be used to construct Bruhat intervals which are the face lattices of duals of stacked polytopes (Proposition 7.2). Stanley [18] conjectured the non-negativity of the cd-indices of a much more general class of Eulerian posets. We show (Theorem 7.4) that if the conjectured non-negativity holds for Bruhat intervals, then the cd-index of any lower Bruhat interval is bounded above by the cd-index of a Boolean algebra. Since the flag h-vectors of Bruhat intervals are non-negative, we are able to prove that the flag h-vectors of lower Bruhat intervals are bounded above by the flag h-vectors of Boolean algebras (Theorem 7.5). The remainder of the paper is organized as follows: We begin with background information on the basic objects appearing in this paper, namely, posets, Coxeter groups, Bruhat order and polytopes in Section 1, CW complexes and PL topology in Section 2 and the cd-index in Section 3. In Section 4, the zipping operation is introduced, and its basic properties are proven. Section 5 contains the proof of the structural recursion. In Section 6 we state and prove the cd-index recursions and apply them to determine the affine span of cd-indices of Bruhat intervals. Section 7 is a discussion of conjectured bounds on the coefficients of the cd-index of a Bruhat interval, including the construction of Bruhat intervals which are isomorphic to the face lattices of dual stacked polytopes. the electronic journal of combinatorics 11 (2004), #R74 2