Class of Infinite Cardinals is Proper Class
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The class of infinite cardinals $\NN’$ is a proper class.
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.
- 1971: Gaisi Takeuti and Wilson M. Zaring: Introduction to Axiomatic Set Theory: $\S 10.43$