# Join in Inclusion Ordered Set

## Theorem

Let $P = \left({X, \subseteq}\right)$ be an inclusion ordered set.

Let $A, B \in X$ such that

$A \cup B \in X$

Then $A \vee B = A \cup B$

## Proof

$A \subseteq A \cup B$ and $B \subseteq A \cup B$

By definition:

$A \cup B$ is upper bound for $\left\{ {A, B}\right\}$

We will prove that

$\forall C \in X: C$ is upper bound for $\left\{ {A, B}\right\} \implies A \cup B \subseteq C$

Let $C \in X$ such that

$C$ is upper bound for $\left\{ {A, B}\right\}$.

By definition of upper bound:

$A \subseteq C$ and $B \subseteq C$

Thus by Union is Smallest Superset:

$A \cup B \subseteq C$

$\Box$

By definition of supremum:

$\sup \left\{ {A, B}\right\} = A \cup B$

Thus by definition of join:

$A \vee B = A \cup B$

$\blacksquare$