Signed Alternating-runs enumeration in Classical Weyl Groups

TL;DR

This paper derives a simplified signed enumeration formula for alternating runs in classical Weyl groups, leading to new insights and applications in permutation enumeration and Coxeter group analysis.

Contribution

It introduces a neat signed enumeration formula for alternating runs, refining previous complex formulas and extending results to type B and D Coxeter groups.

Findings

A simplified signed enumeration formula for permutations in symmetric groups.

Refinement of Wilf's result on divisibility of alternating-runs polynomial.

Enumeration of alternating permutations in classical Weyl groups.

Abstract

The alternating-runs polynomial enumerates alternating runs in the symmetric group. There are three formulae for the number of permutations, $R_{n,k}$ in $\mathfrak{S}_n$ with $k$ alternating runs, but all of them are complicated. We show that when enumerated with sign taken into account, one gets a {\it neat formula}. As a consequence, we get a near refinement of a result of Wilf on the exponent of $(1+t)$ when it divides the alternating-runs polynomial in the alternating group $\mathcal{A}_n$. Other applications include a moment-type identity and enumeration of alternating permutations in $\mathcal{A}_n$. Similar results are obtained for the type B and type D Coxeter groups.