On Graded Semigroup $C^*$-Algebras and Hilbert Modules

Reduced semigroup $C^*$-algebras for arbitrary cancellative semigroups are studied. It is proved that if there exists a semigroup epimorphism from a semigroup to an arbitrary group $G$, then the corresponding semigroup $C^*$-algebra is topologically $G$-graded. It is also demonstrated that if the group is finite, then the graded semigroup $C^*$-algebra has the structure of a projective Hilbert $C^*$-module.