Given a topological space **X** and a finite cover **X** = $\cup X_i$, one can define Cech cohomology of a sheaf of abelian groups **F** with respect to the cover $\{X_i\}$ in two different ways:

- (Ordered): The kth term of the Cech complex is $\bigoplus_{i_1 < \ldots < i_k} \Gamma(X_{i_1} \cap \ldots \cap X_{i_k}, F)$.
- (Unordered): The kth term of the Cech complex is $\bigoplus_{i_1, \ldots , i_k} \Gamma(X_{i_1} \cap \ldots \cap X_{i_k}, F)$.

In particular, the second description involves repetition and is non-zero in every degree. These two descriptions give isomorphic cohomology (the first maps you try to write down will likely be homotopy equivalences).

**Question**: Is there a canonical reference for this fact?