The inversion number statistic for inversion sequences

Abstract

Inversion sequences, also known as subexcedant sequences, form a fundamental class of objects in enumerative combinatorics. In this paper, we study the joint distribution of five statistics on inversion sequences. While several statistics on inversion sequences have been extensively investigated, our contribution is to introduce the inversion number statistic, originally defined for permutations, into the context of inversion sequences. As special cases, we recover classical permutation statistics, including the Stirling, Mahonian and Eulerian distributions, as well as the Catalan and Narayana numbers. Somewhat unexpectedly, our specializations also include the number of involutions in the symmetric group. Our study arises from a $q$-analog of Comtet's expansion formula obtained by substituting the classical derivative operator $D$ with the $q$-derivative operator $D_q$.