Fringe trees of Patricia tries and compressed binary search trees

Svante Janson

Published 2024-05-02Version 1

We study the distribution of fringe trees in Patricia tries and compressed binary search trees; both cases are random binary trees that have been compressed by deleting vertices of outdegree 1 so that they are random full binary trees. The main results are central limit theorems for the number of fringe trees of a given type, which imply quenched and annealed limit results for the fringe tree distribution; for Patricia tries, this is complicated by periodic oscillations in the usual manner. We also consider extended fringe trees. The results are derived from earlier results for uncompressed tries and binary search trees. In the case of compressed binary search trees, it seems difficult to give a closed formula for the asymptotic fringe tree distribution, but we provide a recursion and give examples.