Cartesian Product of Sets is Set

Theorem
Let $V$ be a basic universe.

Let $A$ and $B$ be sets in $V$.

Then their cartesian product $A \times B$ is also a set.

Proof
Let $A$ and $B$ be sets in $V$.

Because $V$ is a basic universe, the basic universe axioms apply.

Hence by the axiom of pairing $\set {A, B}$ is a set.

Then by the axiom of unions $\bigcup \set {A, B}$ is also a set.

We have that $A \cup B = \bigcup \set {A, B}$.

By the axiom of powers $\powerset {A \cup B}$ is a set.

Therefore, also by the axiom of powers, so is $\powerset {\powerset {A \cup B} }$.

It remains to be shown that $A \times B$ is a subclass of $\powerset {\powerset {A \cup B} }$.

Let $x \in A \times B$.

Then:
 * $x = \tuple {a, b}$

for some $a \in A$ and $b \in B$.

By definition of ordered pair:
 * $x = \set {\set a, \set {a, b} }$

From Set is Subset of Union:
 * $A \subseteq A \cup B$ and $B \subseteq A \cup B$

Hence by definition of subset:
 * $a \in A \cup B$

and:
 * $b \in A \cup B$

Thus:
 * $\set a \subseteq A \cup B$

and:
 * $\set {a, b} \subseteq A \cup B$

Thus by definition of power set:
 * $\set a \in \powerset {A \cup B}$

and:
 * $\set {a, b} \in \powerset {A \cup B}$

Hence:
 * $\set {\set a, \set {a, b} } \subseteq \powerset {A \cup B}$

That is:
 * $\tuple {a, b} \subseteq \powerset {A \cup B}$

and so:
 * $x \subseteq \powerset {A \cup B}$

which means:
 * $x \in \powerset {\powerset {A \cup B} }$

Hence by definition of subclass:


 * $A \times B \subseteq \powerset {\powerset {A \cup B} }$

We have that $\powerset {\powerset {A \cup B} }$ is a set.

By the it follows that $A \times B$ is also a set.