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

We consider the discrete-time threshold-$θ\ge 2$ contact process on a random r-regular graph on n vertices. In this process, a vertex with at least θoccupied neighbors at time t will be occupied at time t+1 with probability p, and vacant otherwise. We show that if $θ\ge 2$ and $r \ge θ+2$, $ε_1$ is small and p is at least $p_1(ε_1)$, then starting from all vertices occupied the fraction of occupied vertices stays above $1-2ε_1$ up to time $\exp(γ_1(r)n)$ with probability at least $1 - \exp(-γ_1(r)n)$. In the other direction, we show that for $p_2 < 1$ there is an $ε_2(p_2)>0$ so that if $p \le p_2$ and the number of occupied vertices in the initial configuration is at most $ε_2(p_2)n$, 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 $ε_2(p_1)$, and allow us to conclude that the process on the r-tree has a first order phase transition.

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