Processing math: 100%

The Stacks project

Lemma 7.25.4. Let \mathcal{C} be a site. Let U \in \mathop{\mathrm{Ob}}\nolimits (\mathcal{C}). The functor j_{U!} gives an equivalence of categories

\mathop{\mathit{Sh}}\nolimits (\mathcal{C}/U) \longrightarrow \mathop{\mathit{Sh}}\nolimits (\mathcal{C})/h_ U^\#

Proof. Let us denote objects of \mathcal{C}/U as pairs (X, a) where X is an object of \mathcal{C} and a : X \to U is a morphism of \mathcal{C}. Similarly, objects of \mathop{\mathit{Sh}}\nolimits (\mathcal{C})/h_ U^\# are pairs (\mathcal{F}, \varphi ). The functor \mathop{\mathit{Sh}}\nolimits (\mathcal{C}/U) \to \mathop{\mathit{Sh}}\nolimits (\mathcal{C})/h_ U^\# sends \mathcal{G} to the pair (j_{U!}\mathcal{G}, \gamma ) where \gamma is the composition of j_{U!}\mathcal{G} \to j_{U!}* with the identification j_{U!}* = h_ U^\# .

Let us construct a functor from \mathop{\mathit{Sh}}\nolimits (\mathcal{C})/h_ U^\# to \mathop{\mathit{Sh}}\nolimits (\mathcal{C}/U). Suppose that (\mathcal{F}, \varphi ) is given. For an object (X, a) of \mathcal{C}/U we consider the set \mathcal{F}_\varphi (X, a) of elements s \in \mathcal{F}(X) which under \varphi map to the image of a \in \mathop{\mathrm{Mor}}\nolimits _\mathcal {C}(X, U) = h_ U(X) in h_ U^\# (X). It is easy to see that (X, a) \mapsto \mathcal{F}_\varphi (X, a) is a sheaf on \mathcal{C}/U. Clearly, the rule (\mathcal{F}, \varphi ) \mapsto \mathcal{F}_\varphi defines a functor \mathop{\mathit{Sh}}\nolimits (\mathcal{C})/h_ U^\# \to \mathop{\mathit{Sh}}\nolimits (\mathcal{C}/U).

Consider also the functor \textit{PSh}(\mathcal{C})/h_ U \to \textit{PSh}(\mathcal{C}/U), (\mathcal{F}, \varphi ) \mapsto \mathcal{F}_\varphi where \mathcal{F}_\varphi (X, a) is defined as the set of elements of \mathcal{F}(X) mapping to a \in h_ U(X). We claim that the diagram

\xymatrix{ \textit{PSh}(\mathcal{C})/h_ U \ar[r] \ar[d] & \textit{PSh}(\mathcal{C}/U) \ar[d] \\ \mathop{\mathit{Sh}}\nolimits (\mathcal{C})/h_ U^\# \ar[r] & \mathop{\mathit{Sh}}\nolimits (\mathcal{C}/U) }

commutes, where the vertical arrows are given by sheafification. To see this1, it suffices to prove that the construction commutes with the functor \mathcal{F} \mapsto \mathcal{F}^+ of Lemmas 7.10.3 and 7.10.4 and Theorem 7.10.10. Commutation with \mathcal{F} \mapsto \mathcal{F}^+ follows from the fact that given (X, a) the categories of coverings of (X, a) in \mathcal{C}/U and coverings of X in \mathcal{C} are canonically identified.

Next, let \textit{PSh}(\mathcal{C}/U) \to \textit{PSh}(\mathcal{C})/h_ U send \mathcal{G} to the pair (j_{U!}^{PSh}\mathcal{G}, \gamma ) where j_{U!}^{PSh}\mathcal{G} the presheaf defined by the formula in Lemma 7.25.2 and \gamma is the composition of j_{U!}^{PSh}\mathcal{G} \to j_{U!}* with the identification j_{U!}^{PSh}* = h_ U (obvious from the formula). Then it is immediately clear that the diagram

\xymatrix{ \textit{PSh}(\mathcal{C}/U) \ar[r] \ar[d] & \textit{PSh}(\mathcal{C})/h_ U \ar[d] \\ \mathop{\mathit{Sh}}\nolimits (\mathcal{C}/U) \ar[r] & \mathop{\mathit{Sh}}\nolimits (\mathcal{C})/h_ U^\# }

commutes, where the vertical arrows are sheafification. Putting everything together it suffices to show there are functorial isomorphisms (j_{U!}^{PSh}\mathcal{G})_\gamma = \mathcal{G} for \mathcal{G} in \textit{PSh}(\mathcal{C}/U) and j_{U!}^{PSh}\mathcal{F}_\varphi = \mathcal{F} for (\mathcal{F}, \varphi ) in \textit{PSh}(\mathcal{C})/h_ U. The value of the presheaf (j_{U!}^{PSh}\mathcal{G})_\gamma on (X, a) is the fibre of the map

\coprod \nolimits _{a' : X \to U} \mathcal{G}(X, a') \to \mathop{\mathrm{Mor}}\nolimits _\mathcal {C}(X, U)

over a which is \mathcal{G}(X, a). This proves the first equality. The value of the presheaf j_{U!}^{PSh}\mathcal{F}_\varphi is on X is

\coprod \nolimits _{a : X \to U} \mathcal{F}_\varphi (X, a) = \mathcal{F}(X)

because given a set map S \to S' the set S is the disjoint union of its fibres. \square

[1] An alternative is to describe \mathcal{F}_\varphi by the cartesian diagram
\vcenter { \xymatrix{ \mathcal{F}_\varphi \ar[r] \ar[d] & {*} \ar[d] \\ \mathcal{F}|_{\mathcal{C}/U} \ar[r] & h_ U|_{\mathcal{C}/U} } } \quad \text{for presheaves and}\quad \vcenter { \xymatrix{ \mathcal{F}_\varphi \ar[r] \ar[d] & {*} \ar[d] \\ \mathcal{F}|_{\mathcal{C}/U} \ar[r] & h_ U^\# |_{\mathcal{C}/U} } }
for sheaves and use that restriction to \mathcal{C}/U commutes with sheafification.

Comments (2)

Comment #6029 by Owen on

To conclude that the functors on the bottom rows are equivalences from the fact that the functors on the top rows are equivalences, I think you need to isolate subcategories of and which are mapped by sheafification fully and essentially surjectively to and , respectively (I use for sheafification). As is a full subcategory of that is obvious, but for the relevent subcategory is the essential image of the (fully faithful) functor that sends to , where .

Comment #6176 by on

OK, I think the argument I had in mind is to show that the functors and constructed in the text of the proof are quasi-inverse. The point of having the commutative diagrams involving presheaves, is that it shows that these functors can be computed by computing the result for presheaves and then sheafifying. Thus it suffices to show that the corresponding functors between presheaf categories are quasi-inverse which is what the proof does. OK?

I would be in the market for a clearer proof of this lemma. Anybody?


Your email address will not be published. Required fields are marked.

In your comment you can use Markdown and LaTeX style mathematics (enclose it like $\pi$). A preview option is available if you wish to see how it works out (just click on the eye in the toolbar).

Unfortunately JavaScript is disabled in your browser, so the comment preview function will not work.

All contributions are licensed under the GNU Free Documentation License.