1. Pure-direct-projective modules
2. Bourbaki, N. (1972). Elements of Mathematics, Commutative Algebra, Advanced Book Program. Reading, MA: Addison-Wesley Publishing Company. Originally published as: Bourbaki, N. (1969). Elements De Mathematique, Algebre Commutative, Hermann, Paris.
3. Classes of extensions and resolutions
4. Adjoint functors and equivalences of subcategories
5. Clark, J. (1998). On purely extending modules. Abelian Groups and Modules: Proceedings of the International Conference in Dublin, Trends Math, Basel: Birkhauser, pp. 353–358.