Multivariate normal limit laws for the numbers of fringe subtrees in $ m $-ary search trees and preferential attachment trees

Abstract

We study fringe subtrees of random $ m $-ary search trees and of preferential attachment trees, by putting them in the context of generalised Pólya urns. In particular we show that for the random $ m $-ary search trees with $ m\leq 26 $ and for the linear preferential attachment trees, the number of fringe subtrees that are isomorphic to an arbitrary fixed tree $ T $ converges to a normal distribution; more generally, we also prove multivariate normal distribution results for random vectors of such numbers for different fringe subtrees. Furthermore, we show that the number of protected nodes in random $m$-ary search trees for $ m\leq 26 $ has asymptotically a normal distribution.