Proposition 52.6.12 (099F)—The Stacks project

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Table of contents
Part 3: Topics in Scheme Theory
Chapter 52: Algebraic and Formal Geometry
Section 52.6: Derived completion on a ringed site
Proposition 52.6.12 (cite)

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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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