Composition semigroups and random stability

Abstract

A random variable X is N-divisible if it can be decomposed into a random sum of N i.i.d. components, where N is a random variable independent of the components; X is N-stable if the components are rescaled copies of X. These N-divisible and N-stable random variables arise in a variety of stochastic models, including thinned renewal processes and subordinated Levy and stable processes. We consider a general theory of N-divisibility and N-stability in the case where $E(N) < \infty$, based on a representation of the probability generating function of N in terms of its limiting Laplace. Stieltjes transform $\mathscr{l}$ We analyze certain topological semigroups of such p.g.f.’s in detail, and on this basis we extend existing characterizations of N-divisible and N-stable laws in terms of $\mathscr{l}$ . We apply the results to the aforementioned stochastic models.