The Stacks project

Proposition 52.6.12. Let $(\mathcal{C}, \mathcal{O})$ be a ringed site. Let $\mathcal{I} \subset \mathcal{O}$ be a finite type sheaf of ideals. There exists a left adjoint to the inclusion functor $D_{comp}(\mathcal{O}) \to D(\mathcal{O})$.

Proof. Let $K \to \mathcal{O}$ in $D(\mathcal{O})$ be as constructed in Lemma 52.6.11. Let $E \in D(\mathcal{O})$. Then $E^\wedge = R\mathop{\mathcal{H}\! \mathit{om}}\nolimits (K, E)$ together with the map $E \to E^\wedge $ will do the job. Namely, locally on the site $\mathcal{C}$ we recover the adjoint of Lemma 52.6.8. This shows that $E^\wedge $ is always derived complete and that $E \to E^\wedge $ is an isomorphism if $E$ is derived complete. $\square$

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