Strong survival and extinction for branching random walks via a new order for generating functions

Abstract: We consider general discrete-time branching random walks on a countable set $X$. According to these processes, a particle at $x\in X$ generates a random number of children and places them at (some of) the sites of $X$, not necessarily independently nor with the same law at different starting vertices $x$.We introduce a new type of stochastic ordering of branching random walks, generalizing the germ order introduced by Hutchcroft in arXiv:2011.06402, which relies on the generating function of the process. We prove that given two branching random walks with law $\mathbf{\mu}$ and $\mathbf{\nu}$ respectively, with $\mathbf{\mu} \ge \mathbf{\nu}$, then in every set where there is extinction according to $\mathbf{\mu}$, there is extinction also according to $\mathbf{\nu}$. Moreover, in every set where there is strong local survival according to $\mathbf{\nu}$, there is strong local survival also according to $\mathbf{\nu}$, provided that the supremum of the global extinction probabilities, for the $\mathbf{\nu}$-process, taken over all starting points $x$, is strictly smaller than 1. New conditions for survival and strong survival for inhomogeneous branching random walks are provided. We also extend a result of Moyal which claims that, under some conditions, the global extinction probability for a branching random walk is the only fixed point of its generating function, whose supremum over all starting coordinates, may be smaller than 1.