# Inverse image functor

In mathematics, specifically in algebraic topology and algebraic geometry, an inverse image functor is a contravariant construction of sheaves; here “contravariant” in the sense given a map $f:X\to Y$ , the inverse image functor is a functor from the category of sheaves on Y to that X. The direct image functor is the primary operation on sheaves, with the simplest definition. The inverse image exhibits some relatively subtle features.

## Definition

Suppose we are given a sheaf ${\mathcal {G}}$ on $Y$ and that we want to transport ${\mathcal {G}}$ to $X$ using a continuous map $f\colon X\to Y$ .

We will call the result the inverse image or pullback sheaf $f^{-1}{\mathcal {G}}$ . If we try to imitate the direct image by setting

$f^{-1}{\mathcal {G}}(U)={\mathcal {G}}(f(U))$ for each open set $U$ of $X$ , we immediately run into a problem: $f(U)$ is not necessarily open. The best we could do is to approximate it by open sets, and even then we will get a presheaf and not a sheaf. Consequently, we define $f^{-1}{\mathcal {G}}$ to be the sheaf associated to the presheaf:

$U\mapsto \varinjlim _{V\supseteq f(U)}{\mathcal {G}}(V).$ (Here $U$ is an open subset of $X$ and the colimit runs over all open subsets $V$ of $Y$ containing $f(U)$ .)

For example, if $f$ is just the inclusion of a point $y$ of $Y$ , then $f^{-1}({\mathcal {F}})$ is just the stalk of ${\mathcal {F}}$ at this point.

The restriction maps, as well as the functoriality of the inverse image follows from the universal property of direct limits.

When dealing with morphisms $f\colon X\to Y$ of locally ringed spaces, for example schemes in algebraic geometry, one often works with sheaves of ${\mathcal {O}}_{Y}$ -modules, where ${\mathcal {O}}_{Y}$ is the structure sheaf of $Y$ . Then the functor $f^{-1}$ is inappropriate, because in general it does not even give sheaves of ${\mathcal {O}}_{X}$ -modules. In order to remedy this, one defines in this situation for a sheaf of ${\mathcal {O}}_{Y}$ -modules ${\mathcal {G}}$ its inverse image by

$f^{*}{\mathcal {G}}:=f^{-1}{\mathcal {G}}\otimes _{f^{-1}{\mathcal {O}}_{Y}}{\mathcal {O}}_{X}$ .

## Properties

• While $f^{-1}$ is more complicated to define than $f_{\ast }$ , the stalks are easier to compute: given a point $x\in X$ , one has $(f^{-1}{\mathcal {G}})_{x}\cong {\mathcal {G}}_{f(x)}$ .
• $f^{-1}$ is an exact functor, as can be seen by the above calculation of the stalks.
• $f^{*}$ is (in general) only right exact. If $f^{*}$ is exact, f is called flat.
• $f^{-1}$ is the left adjoint of the direct image functor $f_{\ast }$ . This implies that there are natural unit and counit morphisms ${\mathcal {G}}\rightarrow f_{*}f^{-1}{\mathcal {G}}$ and $f^{-1}f_{*}{\mathcal {F}}\rightarrow {\mathcal {F}}$ . These morphisms yield a natural adjunction correspondence:
$\mathrm {Hom} _{\mathbf {Sh} (X)}(f^{-1}{\mathcal {G}},{\mathcal {F}})=\mathrm {Hom} _{\mathbf {Sh} (Y)}({\mathcal {G}},f_{*}{\mathcal {F}})$ .

However, the morphisms ${\mathcal {G}}\rightarrow f_{*}f^{-1}{\mathcal {G}}$ and $f^{-1}f_{*}{\mathcal {F}}\rightarrow {\mathcal {F}}$ are almost never isomorphisms. For example, if $i\colon Z\to Y$ denotes the inclusion of a closed subset, the stalk of $i_{*}i^{-1}{\mathcal {G}}$ at a point $y\in Y$ is canonically isomorphic to ${\mathcal {G}}_{y}$ if $y$ is in $Z$ and $0$ otherwise. A similar adjunction holds for the case of sheaves of modules, replacing $i^{-1}$ by $i^{*}$ .