# Subset implies Cardinal Inequality

## Theorem

Let $S$ and $T$ be sets such that $S \subseteq T$.

Furthermore, let:

- $T \sim \card T$

where $\card T$ denotes the cardinality of $T$.

Then:

- $\card S \le \card T$

## Proof

For the proof:

- the ordering relation $\le$ for ordinals

and

- the subset relation $\subseteq$

shall be used interchangeably.

Let $f: T \to \card T$ be a bijection.

It follows that $f \restriction_S : S \to \card T$ is an injection.

The image of $S$ under $f$ is a subset of $\card T$ and thus is a subset of an ordinal.

By Unique Isomorphism between Ordinal Subset and Unique Ordinal, there is a unique mapping $\phi$ and a unique ordinal $x$ such that $\phi: x \to f \sqbrk S$ is an order isomorphism.

It follows that $S \sim x$ by the definition of order isomorphism.

Furthermore, $\phi$ is a strictly increasing mapping from ordinals to ordinals.

\(\ds y\) | \(\in\) | \(\ds x\) | ||||||||||||

\(\ds \leadsto \ \ \) | \(\ds y\) | \(\le\) | \(\ds \map \phi y\) | Strictly Increasing Ordinal Mapping Inequality | ||||||||||

\(\ds \) | \(\in\) | \(\ds f \sqbrk S\) | Definition of $\phi$ | |||||||||||

\(\ds \) | \(\subseteq\) | \(\ds \card T\) | Image Preserves Subsets | |||||||||||

\(\ds \leadsto \ \ \) | \(\ds y\) | \(\in\) | \(\ds \card T\) | Cardinal Number is Ordinal |

Therefore, $y \in x \implies y \in \card T$ and $x \le \card T$ by the definition of subset.

But $\card S \le x$ by Cardinal Number Less than Ordinal.

So $\card S \le \card T$ by the fact that Subset Relation is Transitive.

$\blacksquare$

## Also see

- Set Equivalent to Cardinal, which requires the axiom of choice.

## Sources

- 1971: Gaisi Takeuti and Wilson M. Zaring:
*Introduction to Axiomatic Set Theory*: $\S 10.22$

- Mizar article CARD_1:11