Steve Awodey の Category Theory を読む シリーズトップ 4.1 Groups in a category Corollary 4.6 4.2 The category of groups is an equivalence relation Corollary 4.11 Cokernels are special coequalizers 4.3 Groups as categories composition of the congruence category is well-defined is a congruence Theorem 4.13 4.4 Finitely presented categories smallest congruence 参考書籍 4.1 Groups in a category Corollary 4.6 任意の が a