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
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