On the largest common subtree of uniform attachment trees

Abstract: We study the largest common subtree of two independent unlabeled uniform attachment trees (also known as random recursive trees). Our main result shows that, when the two trees have $n$ vertices each, their largest common subtree has at least $n<sup>{0.83}$ vertices with high probability. This is obtained by starting with the common subtree induced by the Ulam--Harris labels in the two trees and improving using local optimization steps. We also give some upper bounds and bounds for general random tree growth models. We leave as an intriguing open question to understand the magnitude of the size of the largest common subtree.