A Note on the γ-Coefficients of the Tree Eulerian Polynomial

We consider the generating polynomial of the number of rooted trees on the set {1, 2, . . . , 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 γ-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, BershteinDotsenko-Khoroshkin, González D’León-Wachs and González D’León related to the free multibracketed Lie algebra and the poset of weighted partitions.