Anisotropic Contact Process on Homogeneous Trees

The existence of a weak survival region is established for the anisotropic symmetric contact process on a homogeneous tree T2d of degree 2d ≥ 4 : For parameter values in a certain connected region of positive Lebesgue measure, the population survives forever with positive probability but ultimately vacates every finite subset of the tree with probability one. In this phase, infection trails must converge to the geometric boundary Ω of the tree. The random subset Λ of the boundary consisting of all ends of the tree in which the infection survives, called the limit set of the process, is shown to have Hausdorff dimension no larger than one half the Hausdorff dimension of the entire geometric boundary. In addition, there is strict inequality at the transition between weak and strong survival except when the contact process is isotropic. It is further shown that in all cases there is a distinguished probability measure μ, supported by Ω, such that the Hausdorff dimension of Λ∩Ωμ, where Ωμ is the set of μ-generic points of Ω, converges to one half the Hausdorff dimension of Ωμ at the phase separation points. Exact formulae for the Hausdorff dimensions of Λ and Λ ∩ Ωμ are obtained. We also prove that the contact process at the transition between extinction and weak survival does not survive. The method developed shows that the contact process at the phase transition to strong survival survives weakly for d ≥ 2.