Lemma 34.6.9. Let $S$ be a scheme. Let $\mathit{Sch}_{syntomic}$ be a big syntomic site containing $S$. The functor $(\textit{Aff}/S)_{syntomic} \to (\mathit{Sch}/S)_{syntomic}$ is special cocontinuous and induces an equivalence of topoi from $\mathop{\mathit{Sh}}\nolimits ((\textit{Aff}/S)_{syntomic})$ to $\mathop{\mathit{Sh}}\nolimits ((\mathit{Sch}/S)_{syntomic})$.

**Proof.**
The notion of a special cocontinuous functor is introduced in Sites, Definition 7.29.2. Thus we have to verify assumptions (1) – (5) of Sites, Lemma 7.29.1. Denote the inclusion functor $u : (\textit{Aff}/S)_{syntomic} \to (\mathit{Sch}/S)_{syntomic}$. Being cocontinuous just means that any syntomic covering of $T/S$, $T$ affine, can be refined by a standard syntomic covering of $T$. This is the content of Lemma 34.6.4. Hence (1) holds. We see $u$ is continuous simply because a standard syntomic covering is a syntomic covering. Hence (2) holds. Parts (3) and (4) follow immediately from the fact that $u$ is fully faithful. And finally condition (5) follows from the fact that every scheme has an affine open covering.
$\square$

## Post a comment

Your email address will not be published.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.

## Comments (0)