On the Axiomatizability of Impossible Futures: Preorder versus Equivalence

We investigate the (in)equational theory of impossible futures semantics over the process algebra BCCSP. We prove that no finite, sound axiomatization for BCCSP modulo impossible futures equivalence is ground-complete. By contrast, we present a finite, sound, ground-complete axiomatization for BCCSP modulo impossible futures preorder. If the alphabet of actions is infinite, then this axiomatization is shown to be omega-complete. If the alphabet is finite, we prove that the in equational theory of BCCSP modulo impossible futures preorder lacks such a finite basis. We also derive non-finite axiomatizability results for nested impossible futures semantics.