# Definition:Ordinal/Definition 3

## Definition

An **ordinal** is a strictly well-ordered set $\left({S, \prec}\right)$ such that:

- $\forall a \in S: S_a = a$

where $S_a$ is the initial segment of $S$ determined by $a$.

From the definition of an initial segment, and Ordering on Ordinal is Subset Relation, we have that:

- $S_a = \left\{{x \in S: x \subsetneqq a}\right\}$

From Initial Segment of Ordinal is Ordinal we have that $S_a$ is itself an ordinal.

## Notation

The class of all ordinals can be found denoted $\operatorname{On}$.

In order to indicate that a set $S$ is an ordinal, this notation is often seen:

- $\operatorname{Ord} S$

whose meaning is:

**$S$ is an ordinal.**

Thus $\operatorname{Ord}$ can be used as a propositional function whose domain is the class of all sets.

According to 1993: Keith Devlin: *The Joy of Sets: Fundamentals of Contemporary Set Theory* (2nd ed.), it is common practice in set theory to use lowercase Greek letters $\alpha,\ \beta,\ \gamma, \ldots$ for ordinals.

## Also known as

An **ordinal** is also known as an **ordinal number**.

For a given well-ordered set $\left({X, \preceq}\right)$, the expression:

- $\operatorname{Ord} \left({X}\right)$

can be used to denote the unique ordinal which is order isomorphic to $\left({X, \preceq}\right)$.

## Also see

- Ordering on Ordinal is Subset Relation where it is shown that $\forall a, b \in S$, the following statements are equivalent:
- $b \prec a$
- $b \subsetneqq a$
- $b \in a$

It is customary to denote the ordering relation between ordinals as $\le$ rather than $\subseteq$ or $\preceq$.

- Results about
**ordinals**can be found here.

## Sources

- 1993: Keith Devlin:
*The Joy of Sets: Fundamentals of Contemporary Set Theory*(2nd ed.) ... (previous) ... (next): $\S 1.7$: Well-Orderings and Ordinals - 2008: Paul Halmos and Steven Givant:
*Introduction to Boolean Algebras*... (previous) ... (next): Appendix $\text{A}$: Set Theory: Natural and Ordinal Numbers