A phase transition in the coming down from infinity of simple exchangeable fragmentation-coagulation processes

Abstract: We consider the class of exchangeable fragmentation-coagulation (EFC) processes where coagulations are multiple and not simultaneous, as in a $\Lambda$-coalescent, and fragmentation dislocates at finite rate an individual block into sub-blocks of infinite size. We call these partition-valued processes, simple EFC processes, and study the question whether such a process, when started with infinitely many blocks, can visit partitions with a finite number of blocks or not. When this occurs, one says that the process comes down from infinity. We introduce two sharp parameters $\theta_{\star}\leq \theta^{\star}\in [0,\infty]$, so that if $\theta^{\star}<1$, the process comes down from infinity and if $\theta_\star>1$, then it stays infinite. We illustrate our result with regularly varying coagulation and fragmentation measures. In this case, the parameters $\theta_{\star},\theta^{\star}$ coincide and are explicit.