A first order phase transition in the threshold-$\theta\ge 2$ contact process on random $r$-regular graphs and $r$-trees

TL;DR

This paper demonstrates a first order phase transition in a threshold contact process on random r-regular graphs and r-trees, revealing a sharp change in long-term behavior depending on the infection probability p.

Contribution

It establishes the existence of a first order phase transition in the threshold-$ heta extgreater=2$ contact process on random r-regular graphs and r-trees, a novel insight into the process's dynamics.

Findings

High initial occupancy sustains large occupied fraction for exponential time.

Low initial occupancy leads to rapid extinction within logarithmic time.

No intermediate quasi-stationary distribution exists between zero and a small positive density.

Abstract

We consider the discrete-time threshold-$\theta \ge 2$ contact process on a random r-regular graph on n vertices. In this process, a vertex with at least \theta occupied neighbors at time t will be occupied at time t+1 with probability p, and vacant otherwise. We show that if $\theta \ge 2$ and $r \ge \theta+2$, $\epsilon_1$ is small and p is at least $p_1(\epsilon_1)$, then starting from all vertices occupied the fraction of occupied vertices stays above $1-2\epsilon_1$ up to time $\exp(\gamma_1(r)n)$ with probability at least $1 - \exp(-\gamma_1(r)n)$. In the other direction, we show that for $p_2 < 1$ there is an $\epsilon_2(p_2)>0$ so that if $p \le p_2$ and the number of occupied vertices in the initial configuration is at most $\epsilon_2(p_2)n$, then with high probability all vertices are vacant at time $C_2(p_2) \log(n)$. These two conclusions imply that on the random r-regular graph there cannot be a quasi-stationary distribution with density of occupied vertices between 0 and $\epsilon_2(p_1)$, and allow us to conclude that the process on the r-tree has a first order phase transition.