A Note on the $\gamma$-Coefficients of the Tree Eulerian Polynomial

Rafael S González D'León

doi:10.37236/5361

Abstract

We consider the generating polynomial of the number of rooted trees on the set $\{1,2,\dots,n\}$ counted by the number of descending edges (a parent with a greater label than a child). This polynomial is an extension of the descent generating polynomial of the set of permutations of a totally ordered $n$-set, known as the Eulerian polynomial. We show how this extension shares some of the properties of the classical one. A classical product formula shows that this polynomial factors completely over the integers. From this product formula it can be concluded that this polynomial has positive coefficients in the $\gamma$-basis and we show that a formula for these coefficients can also be derived. We discuss various combinatorial interpretations of these coefficients in terms of leaf-labeled binary trees and in terms of the Stirling permutations introduced by Gessel and Stanley. These interpretations are derived from previous results of Liu, Dotsenko-Khoroshkin, Bershtein-Dotsenko-Khoroshkin, González D'León-Wachs and Gonzláez D'León related to the free multibracketed Lie algebra and the poset of weighted partitions.

Highlights

A labeled rooted tree T on the set [n] := {1, 2, · · ·, n} is a tree whose nodes or vertices are the elements of [n] and such that one of its nodes has been distinguished and called the root
We consider the generating polynomial of the number of rooted trees on the set {1, 2, . . . , n} counted by the number of descending edges. This polynomial is an extension of the descent generating polynomial of the set of permutations of a totally ordered n-set, known as the Eulerian polynomial
A classical product formula shows that this polynomial factors completely over the integers

Summary

Introduction

A labeled rooted tree T on the set [n] := {1, 2, · · · , n} is a tree whose nodes or vertices are the elements of [n] and such that one of its nodes has been distinguished and called the root. We consider the generating polynomial of the number of rooted trees on the set {1, 2, . Sn with the set of rooted trees on [n] that have n − 1 internal nodes, each of them having a unique child (and so containing a unique leaf).

Results

Conclusion