Condition for Set Equivalent to Cardinal Number

Theorem

Let $S$ be a set.

Let $\card S$ denote the cardinality of $S$.

That is, let $\card S$ be the intersection of all ordinals equivalent to $S$.

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Note that in the absence of the Axiom of Choice, $\card S$ may be the class of all sets.

Then the following are equivalent:

$(1): \quad S \sim \card S$

$(2): \quad \exists x \in \On: S \sim x$

$(3): \quad \exists x \in \On: \exists y: \paren {y \subseteq x \land S \sim y}$

where $A \sim B$ means that there is a bijection from $A$ onto $B$, and the quantification over $y$ is unbounded (so $y$ may be any set).

Proof

$2 \implies 1$

If $\exists x \in \On: S \sim x$, then by Class of All Ordinals is Well-Ordered by Subset Relation there is a smallest ordinal $x$ such that $S \sim x$.

This smallest ordinal $x$ is the cardinal number of $S$, by definition.

$\Box$

$3 \implies 2$

Suppose that $y \subseteq x$ and $S \sim y$ for some ordinal $x$.

Since $y \subseteq x$, it follows that $y \sim z$ for some $z \in \On$ by Unique Isomorphism between Ordinal Subset and Unique Ordinal.

Therefore, by Set Equivalence behaves like Equivalence Relation, $S \sim z$.

$\Box$

$1 \implies 3$

Suppose that $3$ is not true.

It follows that $S \not \sim x$ for any ordinal $x$.

But $S \not \sim \mathbb U$, so $S \not \sim \card S$ by the definition of cardinal number.

The result follows by contraposition.

$\blacksquare$

Sources

• 1971: Gaisi Takeuti and Wilson M. Zaring: Introduction to Axiomatic Set Theory: $\S 10.9$