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arXiv · 2511.05676

Restricted inversion polynomials

Jeongwon Lee·Nathan Lesnevich·Martha Precup
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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.

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Jeongwon Lee, Nathan Lesnevich, Martha Precup. 2025-11-07. Restricted inversion polynomials. https://arxiv.org/abs/2511.05676

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