Invariance and attraction properties of Galton-Watson trees

TL;DR

This paper characterizes invariants and attractors of Galton-Watson trees under Horton pruning, revealing specific invariant measures and their attraction domains based on offspring distribution tail behavior.

Contribution

It identifies the class of invariant measures for Galton-Watson trees under Horton pruning and describes their attraction domains and properties.

Findings

Invariant measures include critical binary and power-tail Galton-Watson trees.

Invariant measures obey Horton law with a specific exponent.

Attraction domains depend on offspring distribution tail behavior.

Abstract

We give a description of invariants and attractors of the critical and subcritical Galton-Watson tree measures under the operation of Horton pruning (cutting tree leaves with subsequent series reduction). Under a regularity condition, the class of invariant measures consists of the critical binary Galton-Watson tree and a one-parameter family of critical Galton-Watson trees with offspring distribution $\{q_k\}$ that has a power tail $q_k\sim Ck^{-(1+1/q_0)}$, where $q_0\in(1/2,1)$. Each invariant measure has a non-empty domain of attraction under consecutive Horton pruning, specified by the tail behavior of the initial Galton-Watson offspring distribution. The invariant measures satisfy the Toeplitz property for the Tokunaga coefficients and obey the Horton law with exponent $R = (1-q_0)^{-1/q_0}$.