Lemma 10.50.3. 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. Let A' denote the subring of K generated by A and x. Since A\subset A' is an integral extension, we see by Lemma 10.36.17 that there is a prime ideal \mathfrak m' \subset A' lying over \mathfrak m. Then A'_{\mathfrak m'} dominates A. Since A is a valuation ring we conclude that A=A'_{\mathfrak m'} and therefore that x\in A. \square
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