Lemma 18.19.8. Let $\mathcal{C}$ be a site. Let $U \in \mathop{\mathrm{Ob}}\nolimits (\mathcal{C})$. Assume that every $X$ in $\mathcal{C}$ has at most one morphism to $U$. Let $\mathcal{F}$ be an abelian sheaf on $\mathcal{C}/U$. The canonical maps $\mathcal{F} \to j_ U^{-1}j_{U!}\mathcal{F}$ and $j_ U^{-1}j_{U*}\mathcal{F} \to \mathcal{F}$ are isomorphisms.

**Proof.**
This is a special case of Lemma 18.16.4 because the assumption on $U$ is equivalent to the fully faithfulness of the localization functor $\mathcal{C}/U \to \mathcal{C}$.
$\square$

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