Remark 2

Tag 0DZN, permanent. Cite it as [Bourbaki, Tag 0DZN].

Algebra, chapter X, § 7. Produit de composition

When $n = 0$ , the exact sequence (4) reads $0 \to N \xrightarrow{f} M \to 0$ , and the associated class is $f^{-1} \in \mathrm{Hom}_A(M, N) = \mathrm{Ext}^0_A(M, N)$ .

Bourbaki puts the proof directly after the statement, so what is above is the statement and its proof together.

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