Almost all trees have quantum symmetry

Abstract

From the work of Erdős and Rényi from 1963, it is known that almost all graphs have no symmetry. In 2017, Lupini, Mančinska, and Roberson proved a quantum counterpart: Almost all graphs have no quantum symmetry. Here, the notion of quantum symmetry is phrased in terms of Banica’s definition of quantum automorphism groups of finite graphs from 2005, in the framework of Woronowicz’s compact quantum groups. Now, Erdős and Rényi also proved a complementary result in 1963: Almost all trees do have symmetry. The crucial point is the almost sure existence of a cherry in a tree. But even more is true: We almost surely have two cherries in a tree—and we derive that almost all trees have quantum symmetry. We give an explicit proof of this quantum counterpart of Erdős and Rényi’s result on trees.