# 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 Equalâ€Ž 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:

 $\ds S_x$ $=$ $\ds \set {z \in S: z \prec x}$ Initial Segment of Ordinal is Ordinal $\ds$ $=$ $\ds \set {z \in S: z \prec y \land z \prec x}$ $\ds$ $=$ $\ds \set {z \in S_y: z \prec x}$ $\ds$ $=$ $\ds \paren {S_y}_x$

we have:

$(2): \quad \paren {g_y \restriction S_x}: S_x \to \paren {\map Z y}_{\map {g_y} x}$ 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, $f: S \to W$ is a bijection.

Also by this result, $f: S \to \struct {W, \subseteq}$ 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:

 $\ds W_{\map Z y}$ $=$ $\ds \set {\map Z x: \map Z x \subset \map Z y}$ $\ds$ $=$ $\ds \set {\map Z x: x \prec y}$ $\ds$ $=$ $\ds \set {\map {g_y} x: x \prec y}$ $\ds$ $=$ $\ds \map {g_y} {S_y}$ $\ds$ $=$ $\ds \map Z y$

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.

$\blacksquare$