Signed Alternating-runs enumeration 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.