Abstract:
We study monoids $S$ over which the class of injective $S$-acts is axiomatizable, complete, and model complete. We prove that, for a countable commutative monoid or a countable group $S$, the class of injective $S$-acts is axiomatizable if and only if $S$ is a finitely generated monoid. We show that there is no nontrivial monoid nor a group the class of injective acts over which is complete, model complete, or categorical.