On rank functions for heaps

Motivated by work of Stembridge, we study rank functions for Viennot's heaps of pieces. We produce a simple and sufficient criterion for a heap to be a ranked poset and apply the results to the heaps arising from fully commutative words in Coxeter groups.Comment: 18 pages AMSTeX, 3 figure

On planar algebras arising from hypergroups

Let $A$ be an associative algebra with identity and with trace. We study the family of planar algebras on 1-boxes that arise from $A$ in the work of Jones, but with the added assumption that the labels on the 1-boxes come from a discrete hypergroup in the sense of Sunder. This construction equips the algebra $P_n^A$ with a canonical basis, \BB_n^A, which turns out to be a ``tabular'' basis. We examine special cases of this construction to exhibit a close connection between such bases and Kazhdan--Lusztig bases of Hecke algebras of types $A$, $B$, $H$ or $I$.Comment: AMSTeX, approx. 32 pages, 11 figure