Inverse image functor

In mathematics, the inverse image functor is a contravariant construction of sheaves. The direct image functor is the primary operation on sheaves, with the simplest definition. The inverse image exhibits some relatively subtle features.

Definition

Suppose given a sheaf $mathcal\{G\}$ on "Y" and that we want to transport $mathcal\{G\}$ to "X" using a continuous map "f" : "X" → "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 can do is to approximate it by open sets, and even then we will get a presheaf, not a sheaf. Consequently we define $f^\{-1\}mathcal\{G\}$ to be the sheaf associated to the presheaf :

:$U\; mapsto\; varinjlim\_\{Vsupseteq\; f(U)\}mathcal\{G\}(V).$("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("F") is just the stalk of "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 : X → 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"_∗, 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"_∗. This implies that there are natural unit and counit morphisms $mathcal\{G\}\; ightarrow\; f\_*f^\{-1\}mathcal\{G\}$ and $f^\{-1\}f\_*mathcal\{F\}\; ightarrow\; mathcal\{F\}$. However, these are "almost never" isomorphisms. For example, if $i\; :\; Z\; ightarrow\; Y$ denotes the inclusion of a closed subset, the stalks 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 $f^\{-1\}$ by $f^*$.

References

* | year=1986. See section II.4.