MARKovian branching population-valued stochastic processes in discrete time are considered, in which the individuals live on a discrete space of sites and an individual a t site x produces, independently of the others, in the next generation a random offspring whose distribution depends on x, whose mean total number is assumed to be one and whose mean number a t site y is denoted by J(x, (y}). It is proved that, provided the MARKOV chain associated with the transition matrix J is null-recurrent, exactly those among the entrance laws for the population-vnlued process are extremal and have a finite mean number of individuals a t any site and time, which tire "of POISSON type", i.e. arise in a natural way from a PoIssoNian remote past. This generalizes a result of LICCETT/PORT [3] on the pure motion case to the case of branching, and also comments on a remark of DYNKIN ([I], p. 110).
IntroductionLet x be a clustering field on a discrete phase space A , i.e. for any a E A , = qg,) is a probability measure on the space M of counting measures (populations) on A , describing the distribution of the offspring population of an individual 6,. By requiring the branching property x(g+y) = x,@) * qY), @, ! P E M , one obtains a brunching dynamics x ,~) , @ E M . In the terminology of DYNKIN [l] (cf. the addendum in his paper), the correspontling branching process in discrete time (which is an M-valued MARKOV chain with transition probability is a superprocess over the seniigroup generated by the kernel J ( a , .) :=s@(.) x{,)(d@), a E A .In this note, we assume the branching to be criticul, i.e. its intensity kernel J is supposed to obey J ( a , A ) = 1 for all a E A . Hefore giving our main result, let us state some notation:A x-entrcince lnw is a sequence (P,JnCE of distributions on M which obeyThe set of x-entrance laws is convex and admits an integral representation over its extremal elements. Any x-entrance law corresponds to a Markovian sequence ( @ n ) n E E of branching populations, and vice versa.