Lemma 26.23.6. Let $j : X \to Y$ be a morphism of schemes. If $j$ is injective on points, then $j$ is separated.

**Proof.**
Let $z$ be a point of $X \times _ Y X$. Then $x = \text{pr}_1(z)$ and $\text{pr}_2(z)$ are the same because $j$ maps these points to the same point $y$ of $Y$. Then we can choose an affine open neighbourhood $V \subset Y$ of $y$ and an affine open neighbourhood $U \subset X$ of $x$ with $j(U) \subset V$. Then $z \in U \times _ V U \subset X \times _ Y X$. Hence $X \times _ Y X$ is the union of the affine opens $U \times _ V U$. Since $\Delta _{X/Y}^{-1}(U \times _ V U) = U$ and since $U \to U \times _ V U$ is a closed immersion, we conclude that $\Delta _{X/Y}$ is a closed immersion (see argument in the proof of Lemma 26.21.2).
$\square$

## 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Your email address will not be published.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 (1)

Comment #3839 by slogan_bot on

There are also: