Abstract Homological Algebra
A close look at much of the earlier material, especially in the last chapter, reveals the strong connection between projectives and injectives. The idea is this: Formulate your result purely in terms of arrows (morphisms), then reverse them. That is, work in the opposite category. Not everything can be done this way, but a surprising amount can.
KeywordsExact Sequence Commutative Diagram Final Object Additive Category Abelian Category
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