Exceptional inverse image functor

Definition

Let f: X → Y be a continuous map of topological spaces or a morphism of schemes. Then the exceptional inverse image is a functor

Rf^!: D(Y) → D(X)

where D(–) denotes the derived category of sheaves of abelian groups or modules over a fixed ring.

It is defined to be the right adjoint of the total derived functor Rf_! of the direct image with compact support. Its existence follows from certain properties of Rf_! and general theorems about existence of adjoint functors, as does the unicity.

The notation Rf^! is an abuse of notation insofar as there is in general no functor f^! whose derived functor would be Rf^!.

Examples and properties

If f: X → Y is an immersion of a locally closed subspace, then it is possible to define

where G is the subsheaf of F of which the sections on some open subset U of Y are the sections s ∈ F(U) whose support is contained in X. The functor f^! is left exact, and the above Rf^!, whose existence is guaranteed by general structural arguments, is indeed the derived functor of this f^!. Moreover f^! is right adjoint to f_!, too.

Slightly more generally, a similar statement holds for any quasi-finite morphism such as an étale morphism.

If f is an open immersion, the exceptional inverse image equals the usual inverse image.