Descents on nonnesting multipermutations

Abstract

Motivated by recent results on quasi-Stirling permutations, which are permutations of the multiset {1,1,2,2,…,n,n} that avoid the “crossing” patterns 1212 and 2121, we consider nonnesting permutations, defined as those that avoid the patterns 1221 and 2112 instead. We show that the polynomial giving the distribution of the number of descents on nonnesting permutations is a product of an Eulerian polynomial and a Narayana polynomial. It follows that, rather unexpectedly, this polynomial is palindromic. We provide bijective proofs of these facts by composing various transformations on Dyck paths, including the Lalanne–Kreweras involution.