# Condition for Woset to be Isomorphic to Ordinal

## Theorem

Let $\struct {S, \preceq}$ be a woset.

Let $\struct {S, \preceq}$ be such that $\forall a \in S$, the initial segment $S_a$ of $S$ determined by $a$ is order isomorphic to some ordinal.

Then $\struct {S, \preceq}$ itself is order isomorphic to an ordinal.

## Proof

For each $a \in S$, let $g_a: S_a \to \map Z a$ be an order isomorphism from $S_a$ to an ordinal $\map Z a$.

By Isomorphic Ordinals are Equal and Order Isomorphism between Wosets is Unique, both $\map Z a$ and $g_a$ are unique.

So this defines a mapping $Z$ on $S$.

Let the image of $Z$ be $W$:

- $W = \set {\map Z a: a \in S}$

Now we define $f: S \to W$ as:

- $\map f a = \map Z a$

Now we show that $x, y, \in S, x \prec y \implies \map Z x \subset \map Z y$.

So, let $x, y, \in S$ such that $x \prec y$. Then:

- $(1) \quad g_x: S_x \cong \map Z x$

Also, because:

\(\displaystyle S_x\) | \(=\) | \(\displaystyle \set {z \in S: z \prec x}\) | Initial Segment of Ordinal is Ordinal | ||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle \set {z \in S: z \prec y \land z \prec x}\) | |||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle \set {z \in S_y: z \prec x}\) | |||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle \paren {S_y}_x\) |

we have:

- $(2): \quad \map {g_y} {S_x}: S_x \to \paren {\map Z y}_{\map {g_y} x}$ is an order isomorphism

So by Isomorphic Ordinals are Equal, we have:

- $(3): \quad \map Z x = \paren {\map Z y}_{\map {g_y} x}$

So, in particular:

- $\map Z x \subset \map Z y$

By this result, $f: X \to W$ is a bijection.

Also by this result, $f: X \to \struct {W, \subseteq}$ is an order isomorphism.

This means that $W$ is well-ordered by $\subseteq$.

Now we show that $W$ is an ordinal.

Let $y \in S$.

Since $\map Z y$ is an ordinal, we have:

- $x \prec y \implies \paren {\map Z y}_{\map {g_y} x} = \map {g_y} x$

So by $(3)$ above, we have:

- $(4) \quad x \prec y \implies \map Z x = \map {g_y} x$

Hence:

\(\displaystyle W_{\map Z y}\) | \(=\) | \(\displaystyle \set {\map Z x: \map Z x \subset \map Z y}\) | |||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle \set {\map Z x: x \prec y}\) | |||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle \set {\map {g_y} x: x \prec y}\) | |||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle \map {g_y} {S_y}\) | |||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle \map Z y\) |

As $y$ is an arbitrary element of $S$, it follows that $\map Z y$ is an arbitrary element of $W$.

So $W$ is an ordinal, as we wanted to prove.

$\blacksquare$

## Sources

- 1993: Keith Devlin:
*The Joy of Sets: Fundamentals of Contemporary Set Theory*(2nd ed.) ... (previous) ... (next): $\S 1$: Naive Set Theory: $\S 1.7$: Well-Orderings and Ordinals: Theorem $1.7.11$