Non-Finite Cardinal is equal to Cardinal Product

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Let $\omega$ denote the minimally inductive set.

Let $x$ be an ordinal such that $x \ge \omega$.


$\card x = \card {x \times x}$

where $\times$ denotes the Cartesian product.


Let $S$ be a set that is equinumerous to its cardinal number.

Let $\card S$ denote the cardinal number of $S$.


$\card S \ge \omega$

where $\omega$ denotes the minimally inductive set.


$\card {S \times S} = \card S$


The proof shall proceed by Transfinite Induction on $x$.


$\forall y \in x: y < \omega \lor \card y = \card {y \times y}$

There are two cases:

Case 1: $\card y = \card x$ for some $y \in x$

If this is so, then:

\(\ds \card x\) \(=\) \(\ds \card y\)
\(\ds \) \(=\) \(\ds \card {y \times y}\) Inductive hypothesis
\(\ds \) \(=\) \(\ds \card {x \times x}\)


Case 2: $\card y < \card x$ for all $y \in x$

We have that either:

$y < \omega$


$\card y = \card {y + 1}$

In either case, we have that:

$\card {y + 1} < \card x$

and therefore:

$y + 1 \in x$

Therefore, $x$ is a limit ordinal.

Let $R_0$ denote the canonical order of $\On^2$.

Let $J_0$ be defined as the unique order isomorphism between $\On^2$ and $\On$ as defined in canonical order.

It follows that:

$\ds \map {J_0} {x \times x} = \bigcup_{y \mathop \in x} \bigcup_{z \mathop \in x} \map {J_0} {y, z}$

But moreover:

\(\ds \map {J_0} {y, z}\) \(=\) \(\ds \map {\in^{-1} } {\map {J_0} {y, z} }\) Definition of Preimage of Element under Mapping
\(\ds \) \(=\) \(\ds \map {J_0} {\map {R_0^{-1} } {y, z} }\) as $J_0$ forms an Order Isomorphism.

Since $J_0$ is a bijection:

\(\ds \card {\map {J_0} {y, z} }\) \(=\) \(\ds \card {\map {J_0} {\map {R_0^{-1} } {y, z} } }\) Equality shown above
\(\ds \) \(=\) \(\ds \card {\map {R_0^{-1} } {y, z} }\) Image of Bijection Cardinal Equality

Take $\max \set {y, z}$.

\(\ds \tuple {a, b}\) \(\in\) \(\ds \map {R_0^{-1} } {y, z}\) by hypothesis
\(\ds \leadstoandfrom \ \ \) \(\ds \tuple {a, b}\) \(R_0\) \(\ds \tuple {y, z}\) Definition of Image of Element under Mapping
\(\ds \leadstoandfrom \ \ \) \(\ds a, b\) \(\le\) \(\ds \max \set {y, z}\) Definition of Canonical Order
\(\ds \leadstoandfrom \ \ \) \(\ds a, b\) \(\in\) \(\ds \max \set {y, z} + 1\) Definition of Successor Set

It follows that:

\(\ds \card {\map {R_0^{-1} } {y, z} }\) \(\le\) \(\ds \card {\max \set {y, z} + 1 \times \max \set {y, z} + 1}\)
\(\ds \) \(=\) \(\ds \card {\max \set {y, z} + 1}\) Inductive hypothesis
\(\ds \) \(<\) \(\ds \card x\) by hypothesis for the case


$\card {\map {R_0^{-1} } {y, z} } < \card x$

Thus by Cardinal Inequality implies Ordinal Inequality:

$\forall y, z \in x: \map {J_0} {y, z} < \card x$

It follows by Supremum Inequality for Ordinals that:

$\map {J_0} {x \times x} \subseteq \card x$


\(\ds \card {x \times x}\) \(=\) \(\ds \card {\map {J_0} {x \times x} }\) Image of Bijection Cardinal Equality
\(\ds \) \(\le\) \(\ds \card x\) Subset of Ordinal implies Cardinal Inequality

But also by Set Less than Cardinal Product:

$\card x \le \card {x \times x}$


$\card x = \card {x \times x}$