Martin boundaries and asymptotic behavior of branching random walks

Let G be an infinite, locally finite graph. We investigate the relation between supercritical, transient branching random walk and the Martin boundary of its underlying random walk. We show results regarding the typical asymptotic directions taken by the particles, and as a consequence we find a new connection between t-Martin boundaries and standard Martin boundaries. Moreover, given a subgraph U we study two aspects of branching random walks on U: when the trajectories visit U infinitely often (survival) and when they stay inside U forever (persistence). We show that there are cases, when U is not connected, where the branching random walk almost surely does not survive in U, but the random walk on G converges to the boundary of U with positive probability. In contrast, the branching random walk can survive with positive probability in U even though the random walk eventually exits U almost surely. We provide several examples and counterexamples.