Anisotropic branching random walks on homogeneous trees

Symmetric branching random walk on a homogeneous tree exhibits a weak survi val phase: For parameter values in a certain interval, the population survi ves forever with positive probability, but, with probability one, eventuall y vacates every finite subset of the tree. In this phase, particle trails m ust converge to the geometric boundary Omega of the tree. The random subset Lambda of the boundary consisting of all ends of the tree in which the pop ulation survives, called the limit set of the process, is shown to have Hau sdorff dimension no larger than one half the Hausdorff dimension of the ent ire geometric boundary. Moreover, there is strict inequality at the phase s eparation point between weak and strong survival except when the branching random walk is isotropic. It is further shown that in all cases there is a distinguished probability measure mu supported by Omega such that the Hausd orff dimension of Lambda boolean AND Omega(mu) , where Omega(mu) is the set of mu-generic points of Omega, converges to one half the Hausdorff dimensi on of Omega(mu) at the phase separation point. Exact formulas are obtained for the Hausdorff dimensions of Lambda and Lambda boolean AND Omega(mu) , a nd it is shown that the log Hausdorff dimension of Lambda has critical expo nent 1/2 at the phase separation point.