Counting permutations by alternating runs via Hetyei-Reiner trees

The generating polynomial of permutations of size $n$, counted by the number of alternating runs, has a root at $-1$ of multiplicity $\lfloor (n-2)\mathrm{/}2\rfloor$ for all $n\ge 2$. This result can be derived by combining the David–Barton formula for Eulerian polynomials with the Foata–Schützenberger $\gamma$–decomposition. More recently, Bóna gave a group–action proof of this phenomenon. In this paper, we present an alternative approach based on the Hetyei–Reiner action on binary trees, which leads to a new combinatorial interpretation of Bóna’s quotient polynomial. Moreover, we extend our analysis to analogous results for permutations of types~$B$ and~$D$. As a by–product of our bijective framework, we also obtain combinatorial proofs of David–Barton–type identities for permutations of types~$A$ and~$B$.