The Stacks project

Lemma 10.122.5. Let $R \to S$ be a finite type ring map. Then $R \to S$ is quasi-finite if and only if for all primes $\mathfrak p \subset R$ the ring $S \otimes _ R \kappa (\mathfrak p)$ is finite over $\kappa (\mathfrak p)$.

Proof. Follows immediately from the more general Lemma 10.122.4. $\square$


Comments (5)

Comment #2822 by Dario Weißmann on

Typos in the proof. It sould read "...of finite type and of dimension . Hence it is finite over for example by Lemma 10.114.4."

Comment #10744 by Rankeya on

Perhaps Lemma 00PM can be re-stated as the following equivalence: Let be a finite type ring map and be a prime ideal of . Then the following are equivalent: 1. is quasi-finite at all primes of lying over . 2. is a finite -algebra. 3. is a finite set.

All three equivalences are essentially noether normalization over a field (since a polynomial ring with at least one variable over a field has infinitely many primes).

Then the connection between quasi-finiteness and Lemma 02ML is clear and one can omit the following sentence before Lemma 02ML: "The following lemma is not quite about quasi-finite ring maps, but it does not seem to fit anywhere else so well."

Comment #10748 by Rankeya on

Sorry, part 2. in my comment above should say is a finite -algebra.


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Your email address will not be published.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 00PM. Beware of the difference between the letter 'O' and the digit '0'.