Spine extremality and its frequency in critical branching random walks
Abstract
We study the generations at which the distinguished spine of a critical branching random walk occupies the spatial minimum. Under finite positive offspring variance and any nonconstant law of independent real edge increments, their number up to generation N is (log N)1+o(1) almost surely and has order log N in probability, with ties included. The same conclusions hold for the unique-minimum event. No spatial moment assumption is needed. The proof combines counting estimates from the spine decomposition with uniform control of extremal multiplicity in an ordinary surviving tree. For a selected minimizer, its one-time probability is the ordinary survival probability qn, and recurrence is equivalent to ∑nqn=∞. We also exhibit a critical infinite-variance offspring family for which the weak-minimum event occurs only finitely often, for every nonconstant increment law. We determine the polynomial order of its last occurrence and, for Gaussian or Laplace increments, prove quantitative eventual separation of the spine from the minimum. The sharp frequency constant and limiting distribution in the nonconstant finite-variance model remain undetermined.
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BibTeX
@misc{zhang2026spine,
title = {{Spine extremality and its frequency in critical branching random walks}},
author = {Haoming Zhang},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.37714},
eprint = {2610.37714},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.37714},
}
RIS
TY - UNPB
TI - Spine extremality and its frequency in critical branching random walks
AU - Haoming Zhang
PY - 2026
DA - 2026/10/27
PB - arXiv
DO - 10.48550/arxiv.2610.37714
UR - https://arxiv.org/abs/2610.37714
AB - We study the generations at which the distinguished spine of a critical branching random walk occupies the spatial minimum. Under finite positive offspring variance and any nonconstant law of independent real edge increments, their number up to generation N is (log N)^1+o(1) almost surely and has order log N in probability, with ties included. The same conclusions hold for the unique-minimum event. No spatial moment assumption is needed. The proof combines counting estimates from the spine decomposition with uniform control of extremal multiplicity in an ordinary surviving tree. For a selected minimizer, its one-time probability is the ordinary survival probability q_n, and recurrence is equivalent to ∑_nq_n=∞. We also exhibit a critical infinite-variance offspring family for which the weak-minimum event occurs only finitely often, for every nonconstant increment law. We determine the polynomial order of its last occurrence and, for Gaussian or Laplace increments, prove quantitative eventual separation of the spine from the minimum. The sharp frequency constant and limiting distribution in the nonconstant finite-variance model remain undetermined.
ER -