Direct image with compact support

In mathematics, in the theory of sheaves the direct image with compact (or proper) support is an image functor for sheaves.

Definition

Let f: X → Y be a continuous mapping of topological spaces, and Sh(–) the category of sheaves of abelian groups on a topological space. The direct image with compact (or proper) support

f_!: Sh(X) → Sh(Y)

sends a sheaf F on X to f_!(F) defined by

f_!(F)(U) := {s ∈ F(f ⁻¹(U)) : f_|supp(s):supp(s)→U is proper},

where U is an open subset of Y. The functoriality of this construction follows from the very basic properties of the support and the definition of sheaves.

Properties

If f is proper, then f_! equals f_∗. In general, f_!(F) is only a subsheaf of f_∗(F)

References

• , esp. section VII.1