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We consider the discrete-time threshold-$\theta \ge 2$ contact process on arandom 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 withprobability 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 $p_1(\epsilon_1)$, thenstarting from all vertices occupied the fraction of occupied vertices staysabove $1-2\epsilon_1$ 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 ofoccupied 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 bea 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 hasa first order phase transition.

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