The Stacks project

Lemma 82.5.19. Let $B \to S$ as in Section 82.2. Let $j : R \to U \times _ B U$ be a set-theoretic pre-equivalence relation. A morphism $\phi : U \to X$ is an orbit space for $R$ if and only if

  1. $\phi \circ s = \phi \circ t$, i.e., $\phi $ is invariant,

  2. the induced morphism $(t, s) : R \to U \times _ X U$ is surjective, and

  3. the morphism $\phi : U \to X$ is surjective.

This characterization applies for example if $j$ is a pre-equivalence relation, or comes from a groupoid in algebraic spaces over $B$, or comes from the action of a group algebraic space over $B$ on $U$.

Proof. Follows immediately from Lemma 82.5.17 part (4). $\square$

Comments (0)

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Your email address will not be published.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.

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 input field. As a reminder, this is tag 049Z. Beware of the difference between the letter 'O' and the digit '0'.