# Definition:Cantor Normal Form

## Definition

Let $x$ be an ordinal.

The Cantor normal form of $x$ is an ordinal summation:

$x = \omega^{a_1} n_1 + \dots + \omega^{a_k} n_k$

where:

$k \in \N$ is a natural number
$\omega$ is the minimal infinite successor set
$\langle a_i \rangle$ is a strictly decreasing finite sequence of ordinals.
$\langle n_i \rangle$ is a finite sequence of finite ordinals

In summation notation:

$x = \displaystyle \sum_{i \mathop = 1}^k \omega^{a_i} n_i$

## Properties

Every ordinal number can be written in Cantor normal form.

Moreover, the Cantor normal form is unique.

The ordinal cannot be written any other way that could still be considered Cantor normal form.

This unique representation is a consequence of the Division Theorem for Ordinals.

Cantor normal form is useful when performing operations like multiplication and exponentiation.

## Source of Name

This entry was named for Georg Cantor.