Enumeration results for alternating tree families

Abstract

We study two enumeration problems for up-down alternating trees, i.e., rooted labelled trees T, where the labels v1,v2,v3,... on every path starting at the root of T satisfy v1 v3.... First we consider various tree families of interest in combinatorics (such as unordered, ordered, d-ary and Motzkin trees) and study the number Tn of different up-down alternating labelled trees of size n. We obtain for all tree families considered an implicit characterization of the exponential generating function T(z) leading to asymptotic results of the coefficients Tn for various tree families. Second we consider the particular family of up-down alternating labelled ordered trees and study the influence of such an alternating labelling to the average shape of the trees by analyzing the parameters label of the root node, degree of the root node and depth of a random node in a random tree of size n. This leads to exact enumeration results and limiting distribution results. © 2010 Elsevier Ltd.