This page is about direct images of sheaves and related subjects. For the set-theoretic operation, see image.
The right adjoint part $f_*$ of any geometric morphism
of toposes is called a direct image.
More generally, pairs of adjoint functors between the categories of sheaves appear in various other setups apart from geometric morphisms of topoi, for instance on abelian categories of quasicoherent sheaves, bounded derived categories of coherent sheaves and the term direct image is used for the right adjoint part of these, too.
Specifically for Grothendieck toposes: a morphism of sites $f : X \to Y$ induces a geometric morphism of Grothendieck toposes
between the categories of sheaves on the sites, with
$p_*$ the direct image
and $p^*$ its left adjoint: the inverse image.
Given a morphism of sites $f : X \to Y$ coming from a functor $f^t : S_Y \to S_X$, the direct image operation on presheaves is the functor
The restriction of this operation to sheaves, which respects sheaves, is the direct image of sheaves
For $X$ a site with a terminal object, let the morphism of sites be the canonical morphism $p : X \to {*}$.
The direct image $p_*$ is the global sections functor;
the inverse image $p^*$ is the constant sheaf functor;
the left adjoint to $p^*$ is $\Pi_0$, the functor of geometric connected components (see homotopy group of an ∞-stack).
See
for the moment.
Let $f:X\to Y$ be a morphism of locally compact topological spaces. Then there exist a unique subfunctor $f_!: Sh(X)\to Sh(Y)$ of the direct image functor $f_*$ such that for any abelian sheaf $F$ over $X$ the sections of $f_!(F)$ over $U^{open}\subset X$ are those sections $s\in f_*(U)= \Gamma(f^{-1}(U),F)$ for which the restriction $supp(s)|f : supp(s)\hookrightarrow U$ is a proper map.
This is called the direct image with compact support.
It follows that $f_!$ is left exact.
Let $p:X\to {*}$ be the map into the one point space. Then for any $F\in Sh(X)$ the abelian sheaf $p_!F$ is the abelian group consisting of sections $s\in \Gamma(X,F)$ such that $supp(s)$ is compact. One writes $\Gamma_c(X,F):= p_! F$ and calls this group a group of sections of $F$ with compact support. If $y\in Y$, then the fiber $(f_! F)_y$ is isomorphic to $\Gamma_c(f^{-1}y,F|_{f^{-1}(y)})$.
Let $f^{-1} \colon Y \to X$ be a morphism of sites. Then the $q$th derived functor $R^q f_\ast$ of the induced direct image functor sends $\mathcal{F} \in Ab(Sh(X_{et}))$ to the sheafification of the presheaf
where on the right we have the degree $q$ abelian sheaf cohomology group with coefficients in the given $\mathcal{F}$.
(e.g. Tamme, I (3.7.1), II (1.3.4), Milne, 12.1).
We have a commuting diagram
where the right vertical morphism is sheafification. Because $(-) \circ f^{-1}$ and $L$ are both exact functors it follows that for $I^\bullet \to \mathcal{F}$ an injective resolution that
e.g.
Last revised on August 6, 2015 at 10:45:39. See the history of this page for a list of all contributions to it.