books.google.com - An Introduction to Homological Algebra discusses the origins of algebraic topology. It also presents the study of homological algebra as a two-stage affair. First, one must learn the language of Ext and Tor and what it describes. Second, one must be able to compute these things, and often, this involves...https://books.google.com/books/about/Introduction_to_Homological_Algebra_85.html?id=yZsHx9mn2OMC&q=commutative+diagram&utm_source=gb-gplus-shareIntroduction to Homological Algebra, 85

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## Introduction to Homological Algebra, 85By Joseph J. Rotman |