A refinement of weak order intervals into distributive lattices

Abstract

In this paper we consider arbitrary intervals in the left weak order on the symmetric group $S_n$. We show that the Lehmer codes of permutations in an interval form a distributive lattice under the product order. Furthermore, the rank-generating function of this distributive lattice matches that of the weak order interval. We construct a poset such that its lattice of order ideals is isomorphic to the lattice of Lehmer codes of permutations in the given interval. We show that there are at least $\left(\lfloor\frac{n}{2}\rfloor\right)!$ permutations in $S_n$ that form a rank-symmetric interval in the weak order.