The Stacks project

Theorem 59.13.2. Let $\mathcal{C}$ be a site and $\mathcal{F}$ a presheaf on $\mathcal{C}$.

1. The rule

   \[ U \mapsto \mathcal{F}^+(U) := \mathop{\mathrm{colim}}\nolimits _{\mathcal{U} \text{ covering of }U} \check H^0(\mathcal{U}, \mathcal{F}) \]

   is a presheaf. And the colimit is a directed one.

2. There is a canonical map of presheaves $\mathcal{F} \to \mathcal{F}^+$.

3. If $\mathcal{F}$ is a separated presheaf then $\mathcal{F}^+$ is a sheaf and the map in (2) is injective.

4. $\mathcal{F}^+$ is a separated presheaf.

5. $\mathcal{F}^\# = (\mathcal{F}^+)^+$ is a sheaf, and the canonical map induces a functorial isomorphism

   \[ \mathop{\mathrm{Hom}}\nolimits _{\textit{PSh}(\mathcal{C})}(\mathcal{F}, \mathcal{G}) = \mathop{\mathrm{Hom}}\nolimits _{\mathop{\mathit{Sh}}\nolimits (\mathcal{C})}(\mathcal{F}^\# , \mathcal{G}) \]

   for any $\mathcal{G} \in \mathop{\mathit{Sh}}\nolimits (\mathcal{C})$.