Lemma 21.21.9. Let $f : (\mathop{\mathit{Sh}}\nolimits (\mathcal{C}), \mathcal{O}_\mathcal {C}) \to (\mathop{\mathit{Sh}}\nolimits (\mathcal{D}), \mathcal{O}_\mathcal {D})$ be a flat morphism of ringed topoi. If $\mathcal{I}^\bullet $ is a K-injective complex of $\mathcal{O}_\mathcal {C}$-modules, then $f_*\mathcal{I}^\bullet $ is K-injective as a complex of $\mathcal{O}_\mathcal {D}$-modules.

**Proof.**
This is true because

\[ \mathop{\mathrm{Hom}}\nolimits _{K(\mathcal{O}_\mathcal {D})}(\mathcal{F}^\bullet , f_*\mathcal{I}^\bullet ) = \mathop{\mathrm{Hom}}\nolimits _{K(\mathcal{O}_\mathcal {C})}(f^*\mathcal{F}^\bullet , \mathcal{I}^\bullet ) \]

by Modules on Sites, Lemma 18.13.2 and the fact that $f^*$ is exact as $f$ is assumed to be flat. $\square$

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