Lemma 6.31.6. Let $X$ be a topological space. Let $j : U \to X$ be the inclusion of an open subset. Consider the functors of restriction and extension by $0$ for abelian (pre)sheaves.

The functor $j_{p!}$ is a left adjoint to the restriction functor $j_ p$ (see Lemma 6.31.1).

The functor $j_!$ is a left adjoint to restriction, in a formula

\[ \mathop{\mathrm{Mor}}\nolimits _{\textit{Ab}(X)}(j_!\mathcal{F}, \mathcal{G}) = \mathop{\mathrm{Mor}}\nolimits _{\textit{Ab}(U)}(\mathcal{F}, j^{-1}\mathcal{G}) = \mathop{\mathrm{Mor}}\nolimits _{\textit{Ab}(U)}(\mathcal{F}, \mathcal{G}|_ U) \]bifunctorially in $\mathcal{F}$ and $\mathcal{G}$.

Let $\mathcal{F}$ be an abelian sheaf on $U$. The stalks of the sheaf $j_!\mathcal{F}$ are described as follows

\[ j_{!}\mathcal{F}_ x = \left\{ \begin{matrix} 0 & \text{if} & x \not\in U \\ \mathcal{F}_ x & \text{if} & x \in U \end{matrix} \right. \]On the category of abelian presheaves of $U$ we have $j_ pj_{p!} = \text{id}$.

On the category of abelian sheaves of $U$ we have $j^{-1}j_! = \text{id}$.

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