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.
Categories: math.CO