Lemma 8.13.1. Let $\mathcal{C}$ be a site. Let $U \in \mathop{\mathrm{Ob}}\nolimits (\mathcal{C})$. Then $j_ U : \mathcal{C}/U \to \mathcal{C}$ is a stack over $\mathcal{C}$ if and only if $h_ U$ is a sheaf.
8.13 Stacks and localization
Let $\mathcal{C}$ be a site. Let $U$ be an object of $\mathcal{C}$. We want to understand stacks over $\mathcal{C}/U$ as stacks over $\mathcal{C}$ together with a morphism towards $U$. The following lemma is the reason why this is easier to do when the presheaf $h_ U$ is a sheaf.
Proof. Combine Lemma 8.6.3 with Categories, Example 4.38.7. $\square$
Assume that $\mathcal{C}$ is a site, and $U$ is an object of $\mathcal{C}$ whose associated representable presheaf is a sheaf. We denote $j : \mathcal{C}/U \to \mathcal{C}$ the localization functor.
Construction A. Let $p : \mathcal{S} \to \mathcal{C}/U$ be a stack over the site $\mathcal{C}/U$. We define a stack $j_!p : j_!\mathcal{S} \to \mathcal{C}$ as follows:
As a category $j_!\mathcal{S} = \mathcal{S}$, and
the functor $j_!p : j_!\mathcal{S} \to \mathcal{C}$ is just the composition $j \circ p$.
We omit the verification that this is a stack (hint: Use that $h_ U$ is a sheaf to glue morphisms to $U$). There is a canonical functor
\[ j_!\mathcal{S} \longrightarrow \mathcal{C}/U \]
namely the functor $p$ which is a $1$-morphism of stacks over $\mathcal{C}$.
Construction B. Let $q : \mathcal{T} \to \mathcal{C}$ be a stack over $\mathcal{C}$ which is endowed with a morphism of stacks $p : \mathcal{T} \to \mathcal{C}/U$ over $\mathcal{C}$. In this case it is automatically the case that $p : \mathcal{T} \to \mathcal{C}/U$ is a stack over $\mathcal{C}/U$.
Lemma 8.13.2. Assume that $\mathcal{C}$ is a site, and $U$ is an object of $\mathcal{C}$ whose associated representable presheaf is a sheaf. Constructions A and B above define mutually inverse (!) functors of $2$-categories
\[ \left\{ \begin{matrix} 2\text{-category of}
\\ \text{stacks over }\mathcal{C}/U
\end{matrix} \right\} \leftrightarrow \left\{ \begin{matrix} 2\text{-category of pairs }(\mathcal{T}, p) \text{ consisting}
\\ \text{of a stack }\mathcal{T}\text{ over }\mathcal{C}\text{ and a morphism}
\\ p : \mathcal{T} \to \mathcal{C}/U\text{ of stacks over }\mathcal{C}
\end{matrix} \right\} \]
Proof. This is clear. $\square$
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