# The Stacks Project

## Tag 03B7

Example 10.155.9. Lemma 10.155.8 does not work if the ring is not Noetherian. For example consider the action of $G = \{+1, -1\}$ on $A = \mathbf{C}[x_1, x_2, x_3, \ldots]$ where $-1$ acts by mapping $x_i$ to $-x_i$. The invariant ring $R = A^G$ is the $\mathbf{C}$-algebra generated by all $x_ix_j$. Hence $R \subset A$ is not finite. But $R$ is a normal domain with fraction field $K = L^G$ the $G$-invariants in the fraction field $L$ of $A$. And clearly $A$ is the integral closure of $R$ in $L$.

The code snippet corresponding to this tag is a part of the file algebra.tex and is located in lines 42624–42636 (see updates for more information).

\begin{example}
Lemma \ref{lemma-Noetherian-normal-domain-finite-separable-extension}
does not work if the ring is not Noetherian.
For example consider the action of $G = \{+1, -1\}$ on
$A = \mathbf{C}[x_1, x_2, x_3, \ldots]$ where $-1$ acts by
mapping $x_i$ to $-x_i$. The invariant ring $R = A^G$ is
the $\mathbf{C}$-algebra generated by all $x_ix_j$. Hence
$R \subset A$ is not finite. But $R$ is a normal domain
with fraction field $K = L^G$ the $G$-invariants in the fraction field
$L$ of $A$. And clearly $A$ is the integral closure of $R$ in
$L$.
\end{example}

There are no comments yet for this tag.

There are also 2 comments on Section 10.155: Commutative Algebra.

## Add a comment on tag 03B7

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).