Definition:Sheaf on Topological Space/Definition 2
Definition
Let $T = \struct {S, \tau}$ be a topological space.
Let $\mathbf C$ be a complete category.
A $\mathbf C$-valued sheaf $\FF$ on $T$ is a $\mathbf C$-valued presheaf such that for all open $U \subset S$ and all open covers $\family {U_i}_{i \mathop \in I}$ of $U$ the morphism:
- $\begin{xy}\xymatrix@L+2mu@+1em{
r : \map F U \ar[r] & \operatorname{eq} \Big(\ds \prod_{i \mathop \in I} \map \FF {U_i} \ar@<-.5ex>[r]_{r_2} \ar@<.5ex>[r]^{r_1} & \ds \prod_{\tuple {i, j} \mathop \in I^2} \map \FF {U_i \cap U_j} \Big) }\end{xy}$ is an isomorphism.
$r_1$ is induced by the universal property of the product by the restriction maps $\operatorname{res}^{U_i}_{U_i \cap U_j} : \map \FF {U_i} \to \map \FF {U_i \cap U_j}$.
$r_2$ is induced by the universal property of the product by the restriction maps $\operatorname{res}^{U_j}_{U_i \cap U_j} : \map \FF {U_j} \to \map \FF {U_i \cap U_j}$.
$r$ is induced by the universal property of the product by the restriction maps $\operatorname{res}^{U}_{U_i} : \map \FF U \to \map \FF {U_i}$ and by the universal property of the equalizer.