# Class of Infinite Cardinals is Proper Class

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## Theorem

The class of infinite cardinals $\NN’$ is a proper class.

## Proof

Aiming for a contradiction, suppose $\NN'$ is a small class.

By Union of Small Classes is Small, $\NN’ \cup \omega$ is a small class.

By definition of the class of infinite cardinals, $\NN \subseteq \NN' \cup \omega$.

But by Axiom of Subsets Equivalents, this means that $\NN$ is a small class.

This contradicts Class of All Cardinals is Proper Class.

Therefore, $\NN’$ is not a small class.

$\blacksquare$

## Sources

- 1971: Gaisi Takeuti and Wilson M. Zaring:
*Introduction to Axiomatic Set Theory*: $\S 10.43$