English

Can one condition a killed random walk to survive?

Probability 2025-09-15 v2

Abstract

We consider the simple random walk on Zd\mathbb{Z}^d killed with probability p(x)p(|x|) at site xx for a function pp decaying at infinity. Due to recurrence in dimension d=2d=2, the killed random walk (KRW) dies almost surely if pp is positive, while in dimension d3d \geq 3 it is known that the KRW dies almost surely if and only if 0rp(r)dr=\int_0^{\infty}rp(r)dr = \infty, under mild technical assumptions on pp. In this paper we consider, for any d2d \geq 2, functions pp for which the KRW dies almost surely and we ask ourselves if the KRW conditioned to survive is well-defined. More precisely, given an exhaustion (ΛR)RN(\Lambda_R)_{R \in \mathbb{N}} of Zd\mathbb{Z}^d, does the KRW conditioned to leave ΛR\Lambda_R before dying converges in distribution towards a limit which does not depend on the exhaustion? We first prove that this conditioning is well-defined for p(r)=o(r2)p(r) = o(r^{-2}), and that it is not for p(r)=min(1,rα)p(r) = \min(1, r^{-\alpha}) for α(14/9,2)\alpha \in (14/9,2). This question is connected to branching random walks and the infinite snake. More precisely, in dimension d=4d=4, the infinite snake is related to the KRW with p(r)(r2log(r))1p(r) \asymp (r^2\log(r))^{-1}, therefore our results imply that the infinite snake conditioned to avoid the origin in four dimensions is well-defined.

Cite

@article{arxiv.2406.12328,
  title  = {Can one condition a killed random walk to survive?},
  author = {Lucas Rey and Augusto Teixeira},
  journal= {arXiv preprint arXiv:2406.12328},
  year   = {2025}
}

Comments

37 pages, 2 figures

R2 v1 2026-06-28T17:09:55.709Z