An **absolute value** of a field $k$ is a function $|\ |:k\to \R_{\ge 0}$ that satisfies:

- $|x|=0$ if and only if $x=0$;
- $|xy| = |x||y|$;
- $|x+y| \le |x|+|y|$.

Absolute values that satisfy the stronger condition $|x+y|\le \max(|x|,|y|)$ are **nonarchimedean**, while those that do not are **archimedean**; the latter arise only in fields of characteristic zero. The **trivial absolute value** assigns 1 to every nonzero element of $k$; it is a nonarchimedean absolute value.

Absolute values $|\ |_1$ and $|\ |_2$ are **equivalent** if there exists a positive real number $c$ such that $|x|_1 = |x|_2^c$ for all $x\in k$; this defines an equivalence relation on the set of absolute values of $k$.

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- Review status: reviewed
- Last edited by John Cremona on 2020-10-10 04:34:40

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