Lemma 21.44.9. Let $(\mathcal{C}, \mathcal{O})$ be a ringed site. Let $\mathcal{E}^\bullet $, $\mathcal{F}^\bullet $ be complexes of $\mathcal{O}$-modules with $\mathcal{E}^\bullet $ strictly perfect. Then the internal hom $R\mathop{\mathcal{H}\! \mathit{om}}\nolimits (\mathcal{E}^\bullet , \mathcal{F}^\bullet )$ is represented by the complex $\mathcal{H}^\bullet $ with terms

\[ \mathcal{H}^ n = \bigoplus \nolimits _{n = p + q} \mathop{\mathcal{H}\! \mathit{om}}\nolimits _\mathcal {O}(\mathcal{E}^{-q}, \mathcal{F}^ p) \]

and differential as described in Section 21.35.

**Proof.**
Choose a quasi-isomorphism $\mathcal{F}^\bullet \to \mathcal{I}^\bullet $ into a K-injective complex. Let $(\mathcal{H}')^\bullet $ be the complex with terms

\[ (\mathcal{H}')^ n = \prod \nolimits _{n = p + q} \mathop{\mathcal{H}\! \mathit{om}}\nolimits _\mathcal {O}(\mathcal{L}^{-q}, \mathcal{I}^ p) \]

which represents $R\mathop{\mathcal{H}\! \mathit{om}}\nolimits (\mathcal{E}^\bullet , \mathcal{F}^\bullet )$ by the construction in Section 21.35. It suffices to show that the map

\[ \mathcal{H}^\bullet \longrightarrow (\mathcal{H}')^\bullet \]

is a quasi-isomorphism. Given an object $U$ of $\mathcal{C}$ we have by inspection

\[ H^0(\mathcal{H}^\bullet (U)) = \mathop{\mathrm{Hom}}\nolimits _{K(\mathcal{O}_ U)}(\mathcal{E}^\bullet |_ U, \mathcal{K}^\bullet |_ U) \to H^0((\mathcal{H}')^\bullet (U)) = \mathop{\mathrm{Hom}}\nolimits _{D(\mathcal{O}_ U)}(\mathcal{E}^\bullet |_ U, \mathcal{K}^\bullet |_ U) \]

By Lemma 21.44.8 the sheafification of $U \mapsto H^0(\mathcal{H}^\bullet (U))$ is equal to the sheafification of $U \mapsto H^0((\mathcal{H}')^\bullet (U))$. A similar argument can be given for the other cohomology sheaves. Thus $\mathcal{H}^\bullet $ is quasi-isomorphic to $(\mathcal{H}')^\bullet $ which proves the lemma.
$\square$

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