Recall the definition of the nerve of an open cover from Čech methods.
If is another cover such that for each , there is a with , then the assignment in this case defines a map
dependent on the choice of for each , but independent ‘up to homotopy’. This gives an inverse system of simplicial sets and homotopy classes of maps.
(i) The classical definition would require that the sets , involved in the simplex were distinct, and that the ordering did not matter. That then gives a simplicial complex rather than a simplicial set. The simplicial complex definition yields something that is smaller but for some calculations is a lot less easy to work with.
(ii) The proof that the homotopy class of the ‘binding’ map from to is independent of the choices made is well known. It shows that any two choices yield ‘contiguous maps’ in as much as the images of a simplex under the two maps, given by the two choices, form two faces of a higher dimensional simplex. This allows an explicit homotopy of simplicial maps to be given.
Taking the simplicial homology groups of the , gives, for each , an inverse system of Abelian groups, which we will denote .
The Čech homology group of the space, , is defined to be
where the limit is taken over all open covers of .
It is to be noted that these groups do not constitute a homology theory in the sense of the Eilenberg-Steenrod axioms as the exactness axiom fails in general. There is a “corrected” theory known under the name strong homology.
See also Čech methods.