# nLab absolute value

### Context

#### Algebra

higher algebra

universal algebra

# Contents

## Terminology

In field theory, what we call an ‘absolute value’ here is often called a ‘valuation’. However, there is also a more general notion of valuation used in field theory, which is what we call ‘valuation’. The notion of absolute value is also used in functional analysis, where it may be called a ‘multiplicative norm’ (rather than merely submultiplicative, as norms on Banach algebras are required to be).

## Definition

For $k$ a rig (typically either a field or at least an integral domain, or else an associative algebra over such), an absolute value on $k$ is a (non-trivial) multiplicative seminorm, or equivalently a finite real-valued valuation.

This means it is a function

${\vert - \vert}\colon k \to \mathbb{R}$

to the real numbers such that for all $x, y \in k$

1. ${\vert x \vert} \geq 0$;

2. ${\vert x \vert} = 0$ precisely if $x = 0$;

3. ${\vert x \cdot y\vert} = {\vert x \vert} {\vert y \vert}$;

4. ${\vert x + y\vert} \leq {\vert x \vert} + {\vert y \vert}$ (the triangle inequality).

If the last trinagle inequality is strengthened to

• ${\vert x + y\vert} \leq max({\vert x \vert}, {\vert y \vert})$

then ${\vert -\vert}$ is called an ultrametric or non-archimedean absolute value. Otherwise it is called archimedean.

Two absolute values ${\vert - \vert}_1$ and ${\vert - \vert}_1$ are called equivalent if for all $x \in k$

$({\vert x \vert}_1 \lt 1) \Leftrightarrow ({\vert x \vert}_2 \lt 1) \,.$

An equivalence class of absolute values is also called a place.

A field equipped with an absolute value which is a complete metric space with respect to the corresponding metric is called a complete field.

## Examples

### Trivial absolute value

Every field admits the trivial absolute value ${\vert- \vert}_0$ defined by

${\vert x \vert}_0 = \left\{ \array{ 0 & if\; x = 0 \\ 1 & otherwise } \right. \,.$

This is non-archimedean.

### On the real and complex numbers

The standard absolute value ${\vert -\vert}_\infty$ on the real numbers is

${\vert x \vert}_{\infty} = \sqrt{x^2} = \left\{ \array{ x & if x \geq 0 \\ - x & otherwise } \right. \,.$

The standard absolute value on the complex numbers is

${\vert x + i y \vert} = \sqrt{x^2 + y^2} \,.$

These standard absolute values are archimedean, and with respect to these standard absolute values, both $\mathbb{R}$ and $\mathbb{C}$ are complete and hence are complete archimedean valued fields. Notice that $\mathbb{R}$ is in addition an ordered field and as such also an archimedean field.

### On the rational numbers

The standard absolute value above restricts to the standard absolute value on the rational numbers

${\vert -\vert}_\infty\colon \mathbb{Q} \to \mathbb{R} \,.$

Moreover, for any prime number $p$ there is a (class of an) absolute value ${\vert -\vert}_p$ on $\mathbb{Q}$ defined by

${\vert \frac{k}{l} p^n\vert}_p = \epsilon^n$

for any $0 \lt \epsilon \lt 1$ and where $\frac{k}{l}p^n$ is the unique such decomposition of a given rational number with $k, l \in \matrhbb{Z}$ not divisible by $p$.

This is called the $p$-adic absolute value. It is non-archimedean. The completion $\mathbb{Q}_p$ of $\mathbb{Q}$ by this is called the field of p-adic numbers, which is therefore a non-archimedean field.

Ostrowski's theorem says that this exhaust the non-trivial absolute values on the rational numbers. Therefore the real numbers and the p-adic numbers are the only possible completions of $\mathbb{Q}$.

### On Laurent power series

The field of Laurent series $k[ [ T] ]$ over a field $k$ is a complete field with respect to the absolute value that sends a series to $\epsilon^n$ for a fixed $0 \lt \epsilon \lt 1$ and with $n$ the lowest integer such that the $n$th coefficient of the series is not $0$.

## References

Section 1.5, 1.6 of

Revised on July 18, 2014 05:03:54 by Urs Schreiber (82.136.246.44)