Nonmonotonic coexistence regions for the two-type Richardson model

In the two-type Richardson model on a graph $\mathcal{G}=(\mathcal{V},\mathcal{E})$, each vertex is at a given time in state $0$, $1$ or $2$. A $0$ flips to a $1$ (resp.\ $2$) at rate $λ_1$ ($λ_2$) times the number of neighboring $1$'s ($2$'s), while $1$'s and $2$'s never flip. When $\mathcal{G}$ is infinite, the main question is whether, starting from a single $1$ and a single $2$, with positive probability we will see both types of infection reach infinitely many sites. This has previously been studied on the $d$-dimensional cubic lattice $\mathbb{Z}^d$, $d\geq 2$, where the conjecture (on which a good deal of progress has been made) is that such coexistence has positive probability if and only if $λ_1=λ_2$. In the present paper examples are given of other graphs where the set of points in the parameter space which admit such coexistence has a more surprising form. In particular, there exist graphs exhibiting coexistence at some value of $\frac{λ_1}{λ_2} \neq 1$ and non-coexistence when this ratio is brought closer to $1$.

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