Presheaf of sets on a topological space
= Presheaf of sets on a topological space
A presheaf of sets on a <topological space> $X$ is a contravariant functor from the inclusion-ordered category $\mathcal O(X)$ of open subsets to $\mathbf{Set}$. Its maps $F(U)\to F(V)$ for $V\subseteq U$ are restriction maps.