# The Stacks Project

## Tag 07XN

Lemma 88.11.3. Let $S$ be a scheme. Let $\mathcal{X}$ be an algebraic stack over $S$. Then the following are equivalent

1. $\mathcal{X}$ is a stack in setoids and $\mathcal{X} \to (\textit{Sch}/S)_{fppf}$ is limit preserving on objects,
2. $\mathcal{X}$ is a stack in setoids and limit preserving,
3. $\mathcal{X}$ is representable by an algebraic space locally of finite presentation.

Proof. Under each of the three assumptions $\mathcal{X}$ is representable by an algebraic space $X$ over $S$, see Algebraic Stacks, Proposition 84.13.3. It is clear that (1) and (2) are equivalent as a functor between setoids is an equivalence if and only if it is surjective on isomorphism classes. Finally, (1) and (3) are equivalent by Limits of Spaces, Proposition 61.3.9. $\square$

The code snippet corresponding to this tag is a part of the file artin.tex and is located in lines 1374–1385 (see updates for more information).

\begin{lemma}
\label{lemma-limit-preserving-algebraic-space}
Let $S$ be a scheme. Let $\mathcal{X}$ be an algebraic stack over $S$.
Then the following are equivalent
\begin{enumerate}
\item $\mathcal{X}$ is a stack in setoids and
$\mathcal{X} \to (\Sch/S)_{fppf}$ is limit preserving on objects,
\item $\mathcal{X}$ is a stack in setoids and limit preserving,
\item $\mathcal{X}$ is representable by an algebraic space
locally of finite presentation.
\end{enumerate}
\end{lemma}

\begin{proof}
Under each of the three assumptions $\mathcal{X}$ is representable
by an algebraic space $X$ over $S$, see Algebraic Stacks, Proposition
\ref{algebraic-proposition-algebraic-stack-no-automorphisms}.
It is clear that (1) and (2) are equivalent as a functor between
setoids is an equivalence if and only if it is surjective on isomorphism
classes. Finally, (1) and (3) are equivalent by
Limits of Spaces, Proposition
\ref{spaces-limits-proposition-characterize-locally-finite-presentation}.
\end{proof}

## Comments (0)

There are no comments yet for this tag.

## Add a comment on tag 07XN

Your email address will not be published.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 lower-right corner).

All contributions are licensed under the GNU Free Documentation License.

In order to prevent bots from posting comments, we would like you to prove that you are human. You can do this by filling in the name of the current tag in the following box. So in case this where tag 0321 you just have to write 0321. Beware of the difference between the letter 'O' and the digit 0.

This captcha seems more appropriate than the usual illegible gibberish, right?