Further remarks on the explicit generating function expression of the invariant measure of critical Galton-Watson Branching Systems

TL;DR

This paper derives a comprehensive explicit generating function for the invariant measure of critical Galton-Watson branching systems with infinite variance, improving existing local expressions and establishing key asymptotic results.

Contribution

It provides a global generating function expression and an improved differential lemma, advancing the understanding of critical branching systems with infinite variance.

Findings

Derived a global generating function for all s in [0,1)

Established an improved differential analogue of the Basic Lemma

Determined the decay rate of the remainder term in the local limit theorem

Abstract

Consider the critical Galton-Watson branching system with infinite variance of the offspring law. We provide an alternative arguments against what Slack~{\cite{Slack68}} did when it seeked for a local expression in the neighborhood of point $1$ of the generating function for invariant measures of the branching system. So, we obtain the global expression for all $s\in[0,1)$ of this generating function. A fundamentally improved version of the differential analogue of the Basic Lemma of the theory of critical branching systems is established. This assertion plays a key role in the formulation of the local limit theorem with explicit terms in the asymptotic expansion of local probabilities. We also determine the decay rate of the remainder term in this expansion.