Renewal Population Dynamics and their Eternal Family Trees

TL;DR

This paper models population dynamics using a simple i.i.d. sequence and explores the structure of the resulting directed random graph, revealing regenerative properties, a unimodular tree structure, and classifications of integers based on their descendants.

Contribution

It introduces a novel connection between population dynamics and unimodular directed trees, characterizing the steady-state structure and classifying integers by their descendant properties.

Findings

Population dynamics are regenerative with a single individual at each epoch.

The directed graph forms a unimodular tree with a unique bi-infinite path.

Integers are classified into ephemeral and successful, forming stationary point processes.

Abstract

Based on a simple object, an i.i.d. sequence of positive integer-valued random variables, $\{a_n\}_{n\in \mathbb{Z}}$, we introduce and study two random structures and their connections. First, a population dynamics, in which each individual is born at time $n$ and dies at time $n+a_n$. This dynamics is that of a D/GI/$\infty$ queue, with arrivals at integer times and service times given by $\{a_n\}_{n\in \mathbb{Z}}$. Second, the directed random graph $T^f$ on $\mathbb{Z}$ generated by the random map $f(n)=n+a_n$. Only assuming $\mathbb{E}[a_0]<\infty$ and $\mathbb{P}[a_0=1]>0$, we show that, in steady state, the population dynamics is regenerative, with one individual alive at each regenerative epochs. We identify a unimodular structure in this dynamics. More precisely, $T^f$ is a unimodular directed tree, in which $f(n)$ is the parent of $n$. This tree has a unique bi-infinite path. Moreover, $T^f$ splits the integers into two categories: ephemeral integers, with a finite number of descendants of all degrees, and successful integers, with an infinite number. Each regenerative epoch is a successful individual such that all integers less than it are its descendants of some order. Ephemeral, successful, and regenerative integers form stationary and mixing point processes on $\mathbb{Z}$.