Complete pure injectivity and endomorphism rings

Abstract

Abstract. It is shown that if M is a finitely presented completely pure injective object in a locally finitely generated Grothendieck category C such that S = Endc M is von Neumann regular, then 5 is semisimple. This is a generalized version of a well-known theorem of Osofsky, which includes also a result of Damiano on PCI-rings. As an application, we obtain a characterization of right hereditary rings with finitely presented injective hull.In [11,12] Osofsky showed that a ring, all of whose cyclic right modules are injective, is semisimple (Artinian). Faith [6] studied the structure of right PCI-rings, i.e., rings whose proper right cyclic modules are injective, and he left open the question of whether right PCI-rings must be right Noetherian. In In this note we prove a general version for Grothendieck categories of Osofsky's theorem [11,12], which includes also the above-mentioned result of Damiano. Furthermore, our arguments provide a simple proof of this result. We show that if Af is a finitely presented completely (pure) A7-injective object in a locally finitely generated Grothendieck category C such that S = Endc M is von Neumann regular, then S is semisimple. Consequently, if Af is a projective completely injective object in C, then Af = ©/6/ A,, where Endc Aj are division rings and the subobjects of Ai are linearly ordered. As an application to rings, we obtain a characterization of the right hereditary rings 7? such that the injective hull E(Rr) is finitely presented. This extends a result of Colby and Rutter [2]. Note that even in the module case, Damiano's arguments cannot be applied for proving our main theorem.