Condition for Cartesian Product Equivalent to Associated Cardinal Number
Theorem
Let $S$ and $T$ be nonempty sets.
Let $\left|{S}\right|$ denote the cardinal number of $S$.
Then:
- $S \times T \sim \left|{S \times T}\right| \iff S \sim \left|{S}\right| \land T \sim \left|{T}\right|$
where $S \times T$ denotes the cartesian product of $S$ and $T$.
Proof
Necessary Condition
If $S \times T \sim \left|{S \times T}\right|$, then there is a mapping $f$ such that:
- $f : S \times T \to \left|{S \times T}\right|$ is a bijection.
Since $f$ is a bijection, it follows that:
- $S$ is equivalent to the image of $S \times \left\{{x}\right\}$ under $f$ where $x \in T$.
This, in turn, is a subset of the ordinal $\left|{S \times T}\right|$.
$\left|{S \times T}\right|$ is an ordinal by Cardinal Number is Ordinal.
By Condition for Set Equivalent to Cardinal Number, it follows that $S \sim \left|{S}\right|$.
Similarly, $T \sim \left|{T}\right|$.
$\Box$
Sufficient Condition
Suppose $f: S \to \left|{S}\right|$ is a bijection and $g: T \to \left|{T}\right|$ is a bijection.
Let $\cdot$ denote ordinal multiplication, while $\times$ shall denote the Cartesian product.
Define the function $F$ to be:
- $\forall x \in S, y \in T: F \left({x, y}\right) = \left|{S}\right| \cdot g \left({y}\right) + f \left({x}\right)$
It follows that $F: S \times T \to \left|{S}\right| \cdot \left|{T}\right|$ is a injection.
By Condition for Set Equivalent to Cardinal Number, it follows that $S \times T \sim \left|{S \times T}\right|$.
$\blacksquare$
Sources
- 1971: Gaisi Takeuti and Wilson M. Zaring: Introduction to Axiomatic Set Theory: $\S 10.15 \ (2)$