Definition 1

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

Algebra, chapter X, § 7. Produit de composition

We call the class associated with the exact sequence (4) the element $\theta$ of $\mathrm{Ext}_A^n(M, N)$ defined by $\theta = (-1)^{n(n+1)/2} \overline{a} = (-1)^{n(n+1)/2} \overline{b}$ .

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

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