Restricted inversion polynomials

TL;DR

This paper introduces a generalization of descent polynomials by counting permutations with specific inversion sets, proves these counts are polynomial, and explores their properties including log-concavity and graded extensions.

Contribution

It generalizes descent polynomials to include permutations with specified inversion subsets and provides explicit polynomial expansions and properties.

Findings

The generalized inversion polynomial $ ext{I}_ extbf{h}(S;n)$ is polynomial.

Two of the polynomial expansions have log-concave coefficients.

A graded generalization of the inversion polynomials is introduced.

Abstract

For a finite subset $I$ of positive integers, the descent polynomial $\mathcal{D}(I;n)$ counts the number of permutations in $S_n$ that have descent set $I$. We generalize descent polynomials by considering permutations with a specific subset $S$ of common inversions called $\mathbf{h}$-inversions, where $\mathbf{h} = (\mathbf{h}(1), \mathbf{h}(2), \ldots )$ is a weakly increasing sequence of positive integers such that $\mathbf{h}(i)> i$. We prove that this more general count, denoted by $\mathcal{I}_\mathbf{h}(S;n)$, is also a polynomial. We give three explicit expansions for $\mathcal{I}_\mathbf{h}(S;n)$, prove the coefficients for two of these expansions are log-concave, and define a graded generalization.