Processing math: 100%

The Stacks project

Lemma 10.79.1. Let R be a ring. Let \varphi : M \to N be a map of R-modules with N a finite R-module. Then we have the equality

\begin{align*} U & = \{ \mathfrak p \subset R \mid \varphi _{\mathfrak p} : M_{\mathfrak p} \to N_{\mathfrak p} \text{ is surjective}\} \\ & = \{ \mathfrak p \subset R \mid \varphi \otimes \kappa (\mathfrak p) : M \otimes \kappa (\mathfrak p) \to N \otimes \kappa (\mathfrak p) \text{ is surjective}\} \end{align*}

and U is an open subset of \mathop{\mathrm{Spec}}(R). Moreover, for any f \in R such that D(f) \subset U the map M_ f \to N_ f is surjective.

Proof. The equality in the displayed formula follows from Nakayama's lemma. Nakayama's lemma also implies that U is open. See Lemma 10.20.1 especially part (3). If D(f) \subset U, then M_ f \to N_ f is surjective on all localizations at primes of R_ f, and hence it is surjective by Lemma 10.23.1. \square


Comments (0)


Your email address will not be published.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.