The Stacks project

Lemma 12.6.3. The construction $(E_1, E_2) \mapsto E_1 + E_2$ above defines a commutative group law on $\mathop{\mathrm{Ext}}\nolimits _\mathcal {A}(B, A)$ which is functorial in both variables.

Proof. Omitted. $\square$

Comments (2)

Comment #8781 by on

S. Mac Lane gave a "proof by smart choice of notation" in his book Homology, III, Theorem 2.1 (second proof). Basically the same proof can also be read in B. Mitchell, Theory of Categories, VII, Theorem 1.5.

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  • 14 comment(s) on Section 12.6: Extensions

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