Lemma 12.7.3. Let \mathcal{A} and \mathcal{B} be abelian categories. Let F : \mathcal{A} \to \mathcal{B} be an exact functor. For every pair of objects A, B of \mathcal{A} the functor F induces an abelian group homomorphism
\mathop{\mathrm{Ext}}\nolimits _\mathcal {A}(B, A) \longrightarrow \mathop{\mathrm{Ext}}\nolimits _\mathcal {B}(F(B), F(A))
which maps the extension E to F(E).
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