Decompositions of CSBPs via Poissonian Intertwining

Abstract

We revisit certain decompositions of continuous-state branching processes (CSBPs), commonly referred to as skeletal decompositions, through the lens of intertwining of semi-groups. Precisely, we associate to a CSBP $X$ with branching mechanism $\psi$ a family of $\mathbb{R}_+\times \mathbb{Z}_+$-valued branching processes $(X^\lambda, L^\lambda)$, indexed by a parameter $\lambda \in (0, \infty)$, that satisfies an intertwining relationship with $X$ through the Poisson kernel with parameter $\lambda x$. The continuous component $X^\lambda$ has the same law as $X$, while the discrete component $L^\lambda$, conditionally on $X^\lambda_t$, has a Poisson distribution with parameter $\lambda X^\lambda_t$. The law of $(X^\lambda, L^\lambda)$ depends on the position of $\lambda$ within $[0, \infty) = [0, \rho) \cup [\rho, \infty)$, where $\rho$ is the largest positive root of $\psi$. When $\lambda \geq \rho$, various well-known results concerning skeleton decompositions are recovered. In the supercritical case ($\rho > 0$), when $\lambda<\rho$, a novel phenomenon arises: a birth term appears in the skeleton, corresponding to a one-unit proportional immigration from the continuous to the discrete component. Along the way, the class of continuous-time branching processes taking values in $\mathbb{R}_+ \times \mathbb{Z}_+$ is constructed.