# Union of Ordinals is Least Upper Bound

## Theorem

Let $A \subset \On$.

That is, let $A$ be a class of ordinals (every member of $A$ is an ordinal).

Then $\bigcup A$, the union of $A$, is the least upper bound of $A$:

$\ds \forall x \in A: x \le A$
$\ds \forall y \in A: y \le x \implies \bigcup A \le x$

## Proof

First, we must show that $\ds \bigcup A$ is an upper bound.

Take any member $a \in A$.

Then by Subset of Union:

$\ds a \subseteq \bigcup A$
$a \le A$

By generalizing for all $a \in A$:

$\ds \forall x \in A: x \le \bigcup A$

Similarly, suppose now that $x$ is an upper bound of $A$.

We shall denote $<$ for ordering on the ordinal numbers.

By Ordering on Ordinal is Subset Relation and Transitive Set is Proper Subset of Ordinal iff Element of Ordinal, $<$ is the same as both $\in$ and $\subsetneq$.

Then:

 $\ds z \in \bigcup A$ $\leadsto$ $\ds \exists y: \paren {z \in y \land y \in A}$ Definition of Union of Set of Sets $\ds$ $\leadsto$ $\ds \exists y: \paren {z \in y \land y < x}$ by hypothesis (as $y \in A$, $y < x$) $\ds$ $\leadsto$ $\ds z \in x$ transitivity of $\in$: see Alternative Definition of Ordinal

Thus, by definition of subset:

$\ds \bigcup A \subseteq x$

Therefore:

$\ds \forall y \in A: y \le x \implies \bigcup A \le x$

$\blacksquare$