The Stacks project

\begin{equation*} \DeclareMathOperator\Coim{Coim} \DeclareMathOperator\Coker{Coker} \DeclareMathOperator\Ext{Ext} \DeclareMathOperator\Hom{Hom} \DeclareMathOperator\Im{Im} \DeclareMathOperator\Ker{Ker} \DeclareMathOperator\Mor{Mor} \DeclareMathOperator\Ob{Ob} \DeclareMathOperator\Sh{Sh} \DeclareMathOperator\SheafExt{\mathcal{E}\mathit{xt}} \DeclareMathOperator\SheafHom{\mathcal{H}\mathit{om}} \DeclareMathOperator\Spec{Spec} \newcommand\colim{\mathop{\mathrm{colim}}\nolimits} \newcommand\lim{\mathop{\mathrm{lim}}\nolimits} \newcommand\Qcoh{\mathit{Qcoh}} \newcommand\Sch{\mathit{Sch}} \newcommand\QCohstack{\mathcal{QC}\!\mathit{oh}} \newcommand\Cohstack{\mathcal{C}\!\mathit{oh}} \newcommand\Spacesstack{\mathcal{S}\!\mathit{paces}} \newcommand\Quotfunctor{\mathrm{Quot}} \newcommand\Hilbfunctor{\mathrm{Hilb}} \newcommand\Curvesstack{\mathcal{C}\!\mathit{urves}} \newcommand\Polarizedstack{\mathcal{P}\!\mathit{olarized}} \newcommand\Complexesstack{\mathcal{C}\!\mathit{omplexes}} \newcommand\Pic{\mathop{\mathrm{Pic}}\nolimits} \newcommand\Picardstack{\mathcal{P}\!\mathit{ic}} \newcommand\Picardfunctor{\mathrm{Pic}} \newcommand\Deformationcategory{\mathcal{D}\!\mathit{ef}} \end{equation*}

Lemma 10.123.2. Let $(R, \mathfrak m_ R) \to (S, \mathfrak m_ S)$ be a local homomorphism of local rings. Assume

  1. $R \to S$ is essentially of finite type,

  2. $\kappa (\mathfrak m_ R) \subset \kappa (\mathfrak m_ S)$ is finite, and

  3. $\dim (S/\mathfrak m_ RS) = 0$.

Then $S$ is the localization of a finite $R$-algebra.

Proof. Let $S'$ be a finite type $R$-algebra such that $S = S'_{\mathfrak q'}$ for some prime $\mathfrak q'$ of $S'$. By Definition 10.121.3 we see that $R \to S'$ is quasi-finite at $\mathfrak q'$. After replacing $S'$ by $S'_{g'}$ for some $g' \in S'$, $g' \not\in \mathfrak q'$ we may assume that $R \to S'$ is quasi-finite, see Lemma 10.122.13. Then by Lemma 10.122.14 there exists a finite $R$-algebra $S''$ and elements $g' \in S'$, $g' \not\in \mathfrak q'$ and $g'' \in S''$ such that $S'_{g'} \cong S''_{g''}$ as $R$-algebras. This proves the lemma. $\square$

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