Lemma 10.50.10. Let $A$ be a valuation ring. Then $A$ is a normal domain.

Proof. Suppose $x$ is in the field of fractions of $A$ and integral over $A$, say $x^{d + 1} + \sum _{i \leq d} a_ i x^ i = 0$. By Lemma 10.50.4 either $x \in A$ (and we're done) or $x^{-1} \in A$. In the second case we see that $x = - \sum a_ i x^{i - d} \in A$ as well. $\square$

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