Counting permutations by alternating runs via Hetyei-Reiner trees

TL;DR

This paper introduces a new combinatorial approach using Hetyei--Reiner trees to analyze permutations by alternating runs, extending results to types B and D, and providing combinatorial proofs of related identities.

Contribution

It presents an alternative combinatorial method based on Hetyei--Reiner action, offering new interpretations and extending results to other permutation types.

Findings

Root at -1 with multiplicity for generating polynomial of permutations

New combinatorial interpretation of Bóna's quotient polynomial

Bijective proofs of identities for permutations of types A and B

Abstract

The generating polynomial of permutations of size $n$, counted by the number of alternating runs, has a root at $-1$ of multiplicity $\lfloor (n-2)/2 \rfloor$ for all $n \ge 2$. This result can be derived by combining the David--Barton formula for Eulerian polynomials with the Foata--Sch\"utzenberger $\gamma$--decomposition. More recently, B\'ona 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 B\'ona'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~$B$.