Cohomological descent

In algebraic geometry, a cohomological descent is, roughly, a "derived" version of a fully faithful descent in the classical descent theory. This point is made precise by the below: the following are equivalent:[1] in an appropriate setting, given a map a from a simplicial space X to a space S,

• ${\displaystyle a^{*}:D^{+}(S)\to D^{+}(X)}$ is fully faithful.
• The natural transformation ${\displaystyle \operatorname {id} _{D^{+}(S)}\to Ra_{*}\circ a^{*}}$ is an isomorphism.

The map a is then said to be a morphism of cohomological descent.[2]

The treatment in SGA uses a lot of topos theory. Conrad's notes gives a more down-to-earth exposition.

See also

• hypercovering, of which a cohomological descent is a generalization

References

1. Conrad, Lemma 6.8. harvnb error: no target: CITEREFConrad (help)
2. Conrad, Definition 6.5. harvnb error: no target: CITEREFConrad (help)

• SGA4 V^bis
• Brian Conrad, Cohomological descent
• P. Deligne, Théorie des Hodge III, Publ. Math. IHES 44 (1975), pp. 6–77.

External links

• http://ncatlab.org/nlab/show/cohomological+descent