Pointlike sets for varieties determined by groups

Abstract

For a variety of finite groups H, let H‾ denote the variety of finite semigroups all of whose subgroups lie in H. We give a characterization of the subsets of a finite semigroup that are pointlike with respect to H‾. Our characterization is effective whenever H has a decidable membership problem. In particular, the separation problem for H‾-languages is decidable for any decidable variety of finite groups H. This generalizes Henckell's theorem on decidability of aperiodic pointlikes.