Definition 15.113.3. Let $A$ be a discrete valuation ring. We say $A$ has mixed characteristic if the characteristic of the residue field of $A$ is $p > 0$ and the characteristic of the fraction field of $A$ is $0$. In this case we obtain an extension of discrete valuation rings $\mathbf{Z}_{(p)} \subset A$ and the absolute ramification index of $A$ is the ramification index of this extension.

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