# Seneta-Heyde norming in the branching random walk

J D Biggins, Andreas E Kyprianou

Research output: Contribution to journalArticlepeer-review

75 Citations (Scopus)

## Abstract

In the discrete-time supercritical branching random walk, there is a Kesten-Stigum type result for the martingales formed by the Laplace transform of the \$n\$th generation positions. Roughly, this says that for suitable values of the argument of the Laplace transform the martingales converge in mean provided an "\$X \log X\$" condition holds. Here it is established that when this moment condition fails, so that the martingale ..converges to zero, it is possible to find a (Seneta-Heyde) renormalization of the martingale that converges in probability to a finite nonzero limit when the process survives. As part of the proof, a Seneta-Heyde renormalization of the general (Crump-Mode-Jagers) branching process is obtained; in this case the convergence holds almost surely. The results rely heavily on a detailed study of the functional equation that the Laplace transform of the limit must satisfy.
Original language English 337-360 24 Annals of Probability 25 1 https://doi.org/10.1214/aop/1024404291 Published - 1997