A Generalized Enumeration of Labeled Trees and Reverse Prüfer Algorithm

Abstract

A {\em leader} of a tree $T$ on $[n]$ is a vertex which has no smaller descendants in $T$. Gessel and Seo showed $$\sum_{T \in \mathcal{T}_n}u^\text{(# of leaders in $T$)} c^\text{(degree of 1 in $T$)}=u P_{n-1}(1,u,cu),$$ which is a generalization of Cayley formula, where ${\mathcal{T}}_n$ is the set of trees on $[n]$ and $$P_n(a,b,c)=c\prod _{i=1}^{n-1}(ia+(n-i)b+c).$$ Using a variation of Prüfer code which is called a {\em RP-code}, we give a simple bijective proof of Gessel and Seo's formula.

5 pages, 3 figures

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