# Definition:Absolute Value

## Contents

## Definition

### Definition 1

Let $x \in \R$ be a real number.

The **absolute value** of $x$ is denoted $\size x$, and is defined using the usual ordering on the real numbers as follows:

- $\size x = \begin{cases} x & : x > 0 \\ 0 & : x = 0 \\ -x & : x < 0 \end{cases}$

### Definition 2

Let $x \in \R$ be a real number.

The **absolute value** of $x$ is denoted $\left\vert{x}\right\vert$, and is defined as:

- $\left\vert{x}\right\vert = +\sqrt {x^2}$

where $\sqrt {x^2}$ is the square root of $x^2$.

## Number Classes

The absolute value function applies to the various number classes as follows:

- Natural numbers $\N$: All elements of $\N$ are greater than or equal to zero, so the concept is irrelevant.
- Integers $\Z$: As defined here.
- Rational numbers $\Q$: As defined here.
- Real numbers $\R$: As defined here.

- Complex numbers $\C$: As $\C$ is not an ordered set, the definition of the absolute value function based upon whether a complex number is greater than or less than zero cannot be applied.

The notation $\cmod z$, where $z \in \C$, is defined as the modulus of $z$ and has a different meaning.

## Ordered Integral Domain

We can go still further back, and consider the general ordered integral domain:

Let $\struct {D, +, \times, \le}$ be an ordered integral domain whose zero is $0_D$.

Then for all $a \in D$, the **absolute value** of $a$ is defined as:

- $\size a = \begin{cases} a & : 0_D \le a \\ -a & : a > 0_D \end{cases}$

## Also known as

The **absolute value of $x$** is sometimes called the **modulus** or **magnitude** of $x$, but note that modulus has a more specialized definition in the domain of complex numbers, and that magnitude has a more specialized definition in the context of vectors.

Some sources refer to it as the **size of $x$**.

## Also see

- Results about
**the absolute value function**can be found here.

### Generalizations

## Technical Note

$\mathsf{Pr} \infty \mathsf{fWiki}$ has a $\LaTeX$ shortcut for the symbol used to denote absolute value:

- The $\LaTeX$ code for \(\size {x}\) is
`\size {x}`

.

If the argument of the `\size`

command is $1$ character, then the braces `{}`

are usually omitted.