On stable-projective and injective-costable decompositions of modules

It is proved that, for a left hereditary ring, an arbitrary left module has a representation in the form of the direct sum of a stable left module and indecomposable projective left modules (if and only if an arbitrary left module has a representation in the form of the direct sum of a stable left module and a projective left module) if and only if the ring is left perfect and right coherent. In that case, the above-mentioned representations are unique up to isomorphism; the latter representation is also functorial. The essential ingredient in the proofs of the above-mentioned statements is a certain purely categorical result. These statements, in particular, imply that, for any principal ideal domain that is not a field, the fundamental theorem on finitely generated modules over it can not be generalized to the case of all modules. Moreover, with the aid of the above-mentioned categorical approach, we give a new proof of the Zheng-Xu He's result asserting that any module of a ring has a unique up to isomorphism injective-costable decomposition if and only if the ring is left hereditary and left Noetherian. The above-mentioned statements, in particular, imply that if the category of left modules over a left hereditary ring is Krull-Schmidt, then the ring is left Artinian. Yet another criterion for a ring to be left hereditary, left perfect and right coherent (resp. left hereditary left Noetherian) found in the paper requires that the pair $(Stable$ $modules$, $Projective$ $modules)$ (resp. $(Injective$ $Modules, Costable$ $Modules)$) of module classes be a pre-torsion theory. This implies that the pair $(Stable$ $modules$, $Projective$ $modules)$ is a torsion theory if and only if the ring is left hereditary and the injective envelope of the ring, viewed as a left module over itself, is projective.