# Cech cohomology

Čech cohomology is a particular type of cohomology in mathematics. It is named for the mathematician Eduard Čech.

## Construction

Let X be a topological space with open cover U={Uα}α, where α is in I, a countable ordered set. We will simplify notation by writing intersections as UαUβ = Uαβ, and so on for higher intersections. For every intersection Uα0···αn there are n + 1 inclusions defined as follows:

i : Uα0···αnUα0···α(i - 1)α(i + 1)···αn

that is, ∂i skips the i th open set. We now define the cochain groups for this cohomology.

Let F be a presheaf. The 0-cochains on U with values in F are functions assigning an element of F(Uα) to every open set Uα. Using the properties of products, we may write

C 0(U,F) = ∏α in I F(Uα).

Then we define the 1-cochains to be elements of

C 1(U,F) = ∏αβ F(Uαβ)

and so on, so that

C i(U,F) = ∏α0···αi F(Uα0···αi ).

Next, the differential. Let δ : C i(U,F)→C i + 1(U,F) be the alternating difference of the F(∂i ):

δ = F(∂0) - F(∂1) + ··· + (-1) i + 1F(∂i ).

One shows that δ² = 0, as required. By taking the cohomology of the chain complex formed we have the Čech cohomology of U with values in the presheaf F. We write

HδC *(U,F) = H *(U,F).

Now, if V is a cover which refines U then there is a well-defined map in cohomology H *(U,F)→H *(V,F) and as such {H *(U,F)}U is a direct system of groups. We call the direct limit of this system the Čech cohomology of X with values in F.

## References

• R. Bott & L. Tu, Differential Forms in Algebraic Topology, Springer Graduate Texts in Mathematics 82, 1982.Template:Math-stub

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