Definition:Cantor Normal Form

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Let $x$ be an ordinal.

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

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


$k \in \N$ is a natural number
$\omega$ is the minimally inductive set
$\sequence {a_i}$ is a strictly decreasing finite sequence of ordinals.
$\sequence {n_i}$ is a finite sequence of finite ordinals

In summation notation:

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


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.

See Ordinal Multiplication via Cantor Normal Form/Limit Base and Ordinal Exponentiation via Cantor Normal Form/Limit Exponents.

Also see

Source of Name

This entry was named for Georg Cantor.