Periodic Modules and Acyclic Complexes

We study the behaviour of modules M that fit into a short exact sequence 0 → M → C → M → 0, where C belongs to a class of modules C, the so-called C-periodic modules. We find a rather general framework to improve and generalize some well-known results of Ben- son and Goodearl and Simson. In the second part we will combine techniques of hereditary cotorsion pairs and presentation of direct limits, to conclude, among other applications, that if M is any module and C is cotorsion, then M will be also cotorsion. This will lead to some meaningful consequences in the category Ch(R) of unbounded chain complexes and in Gorenstein homological algebra. For example we show that every acyclic complex of cotor- sion modules has cotorsion cycles, and more generally, every map F → C where C is a complex of cotorsion modules and F is an acyclic complex of flat cycles, is null-homotopic. In other words, every complex of cotorsion modules is dg-cotorsion.