Extensions of Modules
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In this chapter we develop the classical theory of module extensions and, in particular, its interpretation in terms of cohomology. As explained in the Introduction, to achieve our intended application it is necessary to avoid injective modules. In consequence a feature of our treatment is the development of the exact sequences of Ext using only projective modules.
KeywordsExact Sequence Commutative Diagram Short Exact Sequence Projective Module Projective Resolution