Lemma 29.45.5. Let $f : X \to Y$ be a morphism of schemes. The following are equivalent:

1. $f$ is a universal homeomorphism, and

2. $f$ is integral, universally injective and surjective.

Proof. Assume $f$ is a universal homeomorphism. By Lemma 29.45.4 we see that $f$ is affine. Since $f$ is clearly universally closed we see that $f$ is integral by Lemma 29.44.7. It is also clear that $f$ is universally injective and surjective.

Assume $f$ is integral, universally injective and surjective. By Lemma 29.44.7 $f$ is universally closed. Since it is also universally bijective (see Lemma 29.9.4) we see that it is a universal homeomorphism. $\square$

## 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Your email address will not be published.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.

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 04DF. Beware of the difference between the letter 'O' and the digit '0'.