The Stacks project

Lemma 37.47.4. Let $\pi : X \to Y$ be a finite morphism. Let $x \in X$ with $y = \pi (x)$ such that $\pi ^{-1}(\{ y\} ) = \{ x\} $. Then

  1. For every neighbourhood $U \subset X$ of $x$ in $X$, there exists a neighbourhood $V \subset Y$ of $y$ such that $\pi ^{-1}(V) \subset U$.

  2. The ring map $\mathcal{O}_{Y, y} \to \mathcal{O}_{X, x}$ is finite.

  3. If $\pi $ is of finite presentation, then $\mathcal{O}_{Y, y} \to \mathcal{O}_{X, x}$ is of finite presentation.

  4. For any quasi-coherent $\mathcal{O}_ X$-module $\mathcal{F}$ we have $\mathcal{F}_ x = \pi _*\mathcal{F}_ y$ as $\mathcal{O}_{Y, y}$-modules.

Proof. The first assertion is purely topological; use that $\pi $ is a continuous and closed map such that $\pi ^{-1}(\{ y\} ) = \{ x\} $. To prove the second and third parts we may assume $X = \mathop{\mathrm{Spec}}(B)$ and $Y = \mathop{\mathrm{Spec}}(A)$. Then $A \to B$ is a finite ring map and $y$ corresponds to a prime $\mathfrak p$ of $A$ such that there exists a unique prime $\mathfrak q$ of $B$ lying over $\mathfrak p$. Then $B_{\mathfrak q} = B_{\mathfrak p}$, see Algebra, Lemma 10.41.11. In other words, the map $A_{\mathfrak p} \to B_{\mathfrak q}$ is equal to the map $A_{\mathfrak p} \to B_{\mathfrak p}$ you get from localizing $A \to B$ at $\mathfrak p$. Thus (2) and (3) follow from simple properties of localization (some details omitted). For the final statement, suppose that $\mathcal{F} = \widetilde M$ for some $B$-module $M$. Then $\mathcal{F} = M_{\mathfrak q}$ and $\pi _*\mathcal{F}_ y = M_{\mathfrak p}$. By the above these localizations agree. Alternatively you can use part (1) and the definition of stalks to see that $\mathcal{F}_ x = \pi _*\mathcal{F}_ y$ directly. $\square$


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Your email address will not be published.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 05B8. Beware of the difference between the letter 'O' and the digit '0'.