To my knowledge, there are three major process algebras that have inspired a vast range of research into formal models of concurrency. These are:CCS and $\pi$-calculus both by Robin MilnerCSP by Tony Hoare andACP by Jan Bergstra and Jan Willem KlopAll three seem to have to this day a quite active following and vast amounts of research has been done on them.What are the key similarities and differences of these approaches?   Why has research in process algebra diverged instead of converged, in the sense that there is no one universal model to unify the field? 
Similarities and differences in major process algebras
logic;concurrency;process algebras