It is proved that every commutative ring whose RD-injective modules are Σ-RD-injective is the product of a pure semi-simple ring and a finite ring. A complete characterization of commutative rings for which each artinian (respectively simple) module is RD-injective, is given. These results can be obtained by using the properties of RD-flat modules and RD-coflat modules which are respectively th...

LetK/Qp be a finite extension and GK = Gal(K̄/K) the Galois group of an algebraic closure K̄. Let F be a finite field of characteristic p, and VF a finite dimensional F-vector space equipped with a continuous action of GK . The study of the deformation theory of Galois representations was initiated by Mazur [Ma], who showed that if VF has no non-trivial endomorphisms, then it admits a universal d...

The theory of protomodular categories provides a simple and general context in which the basic theorems needed in homological algebra of groups, rings, Lie algebras and other non-abelian structures can be proved [2] [3] [4] [5] [6] [7] [9] [20]. An interesting aspect of the theory comes from the fact that there is a natural intrinsic notion of normal monomorphism [4]. Since any internal reflexi...