Grothendieck Universe is Closed under Mappings
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Theorem
Let $\mathbb U$ be a Grothendieck universe.
Let $u, v \in \mathbb U$.
Let $f: u \to v$ be a mapping realized as a relation consisting of ordered pairs in Kuratowski formalization.
Then $f \in \mathbb U$.
Proof
Let $u \times v$ be the finite cartesian product of $u$ and $v$ realized as a set of ordered pairs in Kuratowski formalization.
By definition of mapping, we have $f \subseteq u \times v$.
Then:
\(\ds u, v\) | \(\in\) | \(\ds \mathbb U\) | ||||||||||||
\(\ds \leadsto \ \ \) | \(\ds u \times v\) | \(\in\) | \(\ds \mathbb U\) | Grothendieck Universe is Closed under Binary Cartesian Product | ||||||||||
\(\ds \leadsto \ \ \) | \(\ds f\) | \(\in\) | \(\ds \mathbb U\) | Grothendieck Universe is Closed under Subset and $f \subseteq u \times v$ |
$\blacksquare$