Čech-to-derived functor spectral sequence
Let be a sheaf on a topological space X. Choose an open cover of X. That is, is a set of open subsets of X which together cover X. Let denote the presheaf which takes an open set U to the qth cohomology of on U, that is, to . For any presheaf , let denote the pth Čech cohomology of with respect to the cover . Then the Čech-to-derived functor spectral sequence is:
If for all finite intersections of a covering the cohomology vanishes, the E2-term degenerates and the edge morphisms yield an isomorphism of Čech cohomology for this covering to sheaf cohomology. This provides a method of computing sheaf cohomology using Čech cohomology. For instance, this happens if is a quasi-coherent sheaf on a scheme and each element of is an open affine subscheme such that all finite intersections are again affine (e.g. if the scheme is separated). This can be used to compute the cohomology of line bundles on projective space.
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