# The Stacks Project

## Tag 02NG

Lemma 28.19.7. Let $f : X \to S$ be a morphism of schemes. Let $s \in S$. Assume that

1. $f$ is locally of finite type, and
2. $f^{-1}(\{s\})$ is a finite set.

Then $X_s$ is a finite discrete topological space, and $f$ is quasi-finite at each point of $X$ lying over $s$.

Proof. Suppose $T$ is a scheme which (a) is locally of finite type over a field $k$, and (b) has finitely many points. Then Lemma 28.15.10 shows $T$ is a Jacobson scheme. A finite Jacobson space is discrete, see Topology, Lemma 5.18.6. Apply this remark to the fibre $X_s$ which is locally of finite type over $\mathop{\rm Spec}(\kappa(s))$ to see the first statement. Finally, apply Lemma 28.19.6 to see the second. $\square$

The code snippet corresponding to this tag is a part of the file morphisms.tex and is located in lines 3318–3328 (see updates for more information).

\begin{lemma}
\label{lemma-finite-fibre}
Let $f : X \to S$ be a morphism of schemes.
Let $s \in S$. Assume that
\begin{enumerate}
\item $f$ is locally of finite type, and
\item $f^{-1}(\{s\})$ is a finite set.
\end{enumerate}
Then $X_s$ is a finite discrete topological space, and
$f$ is quasi-finite at each point of $X$ lying over $s$.
\end{lemma}

\begin{proof}
Suppose $T$ is a scheme which (a) is locally of finite type
over a field $k$, and (b) has finitely many points. Then
Lemma \ref{lemma-ubiquity-Jacobson-schemes} shows $T$ is a
Jacobson scheme. A finite Jacobson space is discrete, see
Topology, Lemma \ref{topology-lemma-finite-jacobson}.
Apply this remark to the fibre $X_s$ which is locally of finite type over
$\Spec(\kappa(s))$ to see the first statement. Finally, apply
Lemma \ref{lemma-quasi-finite-at-point-characterize} to see the second.
\end{proof}

There are no comments yet for this tag.

## Add a comment on tag 02NG

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 lower-right corner).