The cohomology of sheaves in the étale topology (cf. Etale topology). It is defined in the standard manner by means of derived functors. Let be a scheme and let be the étale topology on . Then the category of sheaves of Abelian groups on is an Abelian category with a sufficient collection of injective objects. The functor of global sections is left exact and its derived functors (where is a sheaf of Abelian groups on ) are called cohomology functors. Here . Similarly, one defines the higher direct images of relative to a morphism ; for them there is an analogue of the Leray spectral sequence. If is a sheaf of non-Abelian groups, one can define the set (see Non-Abelian cohomology).
The most important results in the theory of étale cohomology are obtained for constructible étale sheaves of Abelian groups. Central among them is the finiteness theorem and the base-change theorem: Let be a proper morphism and let be a constructible sheaf on . Then the sheaves are constructible and the stalk of at a geometric point is isomorphic to the cohomology group of the stalk . Similar theorems are true for any morphism of finite type, provided that one uses cohomology with compact support.
If is an algebraic variety over an algebraically closed field, then for any constructible sheaf on the cohomology groups with compact support are finite and vanish for . If, in addition, is an affine variety, then for .
For varieties over the field of complex numbers the étale cohomology of constructible sheaves is the same as the classical cohomology with values in these sheaves. The specialization theorem for a smooth morphism holds: Suppose that is a smooth proper morphism of schemes and that the integer is invertible on . Then the sheaves are locally constant on .
For étale cohomology there is an analogue of Poincaré duality (see Duality in algebraic geometry) and the Künneth formula. Every algebraic cycle of codimension gives a cohomology class in dimension , which makes it possible to construct a theory of Chern classes (cf. Chern class).
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Etale cohomology. Encyclopedia of Mathematics. URL: http://www.encyclopediaofmath.org/index.php?title=Etale_cohomology&oldid=23823