Let $\mathscr{F}$ be a presheaf of abelian groups on some topological space $X$. We say that $\mathscr{F}$ is **locally constant** if there exists an open cover $\mathcal{U}$ of $X$ (i.e. $X=\bigcup_{U\in\mathcal{U}}\ U$ and the elements of $\mathcal{U}$ are open) such that, for all $U\in\mathcal{U}$ and for all $P\in U$, we have $\mathscr{F}(U)=\mathscr{F}_P$.

The task is to prove that for every $U\in\mathcal{U}$ and every connected subset $V\subseteq U$, the composite

$\mathscr{F}(U) \xrightarrow{\ \text{restriction}\ } \mathscr{F}(V) \xrightarrow{\ \text{sheafification}\ } \mathscr{F}^+(V)$

is an isomorphism. This is an exercise in the Book on Algebraic Topology by Spanier. A fellow student asked me about it because he needs the result, but we were unable to figure it out.