Characterizations of projective and -projective semimodules.
We characterize semiperfect modules, semiperfect rings, and perfect rings using locally projective covers and generalized locally projective covers, where locally projective modules were introduced by Zimmermann-Huisgen and generalized locally projective covers are adapted from Azumaya’s generalized projective covers.
Let be a ring. A subclass of left -modules is called a weak torsion class if it is closed under homomorphic images and extensions. Let be a weak torsion class of left -modules and a positive integer. Then a left -module is called -finitely generated if there exists a finitely generated submodule such that ; a left -module is called -presented if there exists an exact sequence of left -modules such that are finitely generated free and is -finitely generated; a left -module...