The probability of random trees being isomorphic

Abstract

We study the fundamental question of how likely it is that two randomly chosen trees are isomorphic to each other for different models of random trees. We show that the probability decays exponentially for rooted labeled trees as well as for Galton--Watson trees with bounded degrees but that this is not true for plane trees, thus providing a counterexample in the general case of Galton--Watson trees without degree restrictions. We also derive limiting distributions for some related parameters: the number of vertices of given degrees in pairs of labeled trees conditioned on being isomorphic, thus showing that they have a different shape than usual labeled trees, as well as the number of labelings and plane representations of P\'olya trees. The results rely on both probabilistic and analytic tools.