Percolation of the contact process on the regular tree

Source abstract

The contact process on the regular tree $\mathbb{T}_d$ when $d\geq 3$ has the two phase transitions of global and of local survival, found by Pemantle and Liggett at values $λ_1$ and $λ_2$. We start the system with every vertex infected and let it relax to what is known as the upper invariant infection. In this stationary state, $λ_p$ is the critical value beyond which the infected vertices can percolate through $\mathbb{T}_d$, and $λ_{p^\complement}$ is the parameter before which the healthy vertices can percolate. We find these are both distinct phase transitions $$0<λ_1<λ_p<λ_2<λ_{p^\complement}<+\infty$$ on $\mathbb{T}_d$ when $d\geq 7$. The most interesting of these comparisons is $λ_1<λ_p$, which we find for all $d\geq 3$. This comparison $λ_1<λ_p$ is a long-standing open question on $\mathbb{Z}^d$ with $d\geq 2$ and was not yet found on any other graphs except where $λ_p$ is infinite.