Successor ordinal
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When defining the ordinal numbers, an absolutely fundamental operation that we can perform on them is a successor operation S to get the next higher one. Using von Neumann's ordinal numbers (the standard ordinals used in set theory), we have, for any ordinal number,
- <math>S(\alpha) = \alpha \cup \{\alpha\}.</math>
Since the ordering on the ordinal numbers α < β if and only if <math>\alpha \in \beta</math>, it is immediate that there is no ordinal number between α and S(α) and it is also clear that α < S(α). An ordinal number which is S(β) for some ordinal β is called a successor ordinal. Ordinals which are neither zero nor successors are called limit ordinals. We can use this operation to define ordinal addition rigorously via transfinite recursion as follows:
- <math>\alpha + 0 = \alpha</math>
- <math>\alpha + S(\beta) = S(\alpha + \beta)</math>
and for a limit ordinal λ
- <math>\alpha + \lambda = \bigcup_{\beta < \lambda} (\alpha + \beta)</math>
In particular, S(α) = α + 1. Multiplication and exponentiation are defined similarly.