The Kesten-Stigum theorem for the one-type Galton-Watson process
gives necessary and sufficient conditions for mean convergence of
the martingale formed by the population size normed by its
expectation. Here, the approach to this theorem pioneered by
Lyons, Pemantle and Peres (1995) is extended to certain kinds of
martingales defined for Galton-Watson processes with a general
type space. Many examples satisfy stochastic domination conditions
on the offspring distributions and suitable domination conditions
combine nicely with general conditions for mean convergence to
produce moment conditions, like the XlogX condition
of the Kesten-Stigum theorem. A general treatment of this
phenomenon is given. The application of the approach to various
branching processes is indicated. However, the main reason for
developing the theory is to obtain martingale convergence results
in a branching random walk that do not seem readily accessible
with other techniques. These results, which are natural extensions
of known results for martingales associated with binary branching
Brownian motion, form the main application.
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