## Tag `08WD`

Chapter 10: Commutative Algebra > Section 10.147: Unramified ring maps

Lemma 10.147.8. Let $R \to S$ be a ring map. The following are equivalent

- $R \to S$ is étale,
- $R \to S$ is flat and G-unramified, and
- $R \to S$ is flat, unramified, and of finite presentation,

Proof.Parts (2) and (3) are equivalent by definition. The implication (1) $\Rightarrow$ (3) follows from the fact that étale ring maps are of finite presentation, Lemma 10.141.3 (flatness of étale maps), and Lemma 10.147.3 (étale maps are unramified). Conversely, the characterization of étale ring maps in Lemma 10.141.7 and the structure of unramified ring maps in Lemma 10.147.5 shows that (3) implies (1). (This uses that $R \to S$ is étale if $R \to S$ is étale at every prime $\mathfrak q \subset S$, see Lemma 10.141.3.) $\square$

The code snippet corresponding to this tag is a part of the file `algebra.tex` and is located in lines 39350–39358 (see updates for more information).

```
\begin{lemma}
\label{lemma-etale-flat-unramified-finite-presentation}
Let $R \to S$ be a ring map. The following are equivalent
\begin{enumerate}
\item $R \to S$ is \'etale,
\item $R \to S$ is flat and G-unramified, and
\item $R \to S$ is flat, unramified, and of finite presentation,
\end{enumerate}
\end{lemma}
\begin{proof}
Parts (2) and (3) are equivalent by definition.
The implication (1) $\Rightarrow$ (3) follows from
the fact that \'etale ring maps are of finite presentation,
Lemma \ref{lemma-etale} (flatness of \'etale maps), and
Lemma \ref{lemma-unramified} (\'etale maps are unramified).
Conversely, the characterization of \'etale ring maps in
Lemma \ref{lemma-characterize-etale}
and the structure of unramified ring maps in
Lemma \ref{lemma-unramified-at-prime}
shows that (3) implies (1). (This uses that $R \to S$
is \'etale if $R \to S$ is \'etale at every prime $\mathfrak q \subset S$,
see Lemma \ref{lemma-etale}.)
\end{proof}
```

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