Join in Inclusion Ordered Set
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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:
- $A \vee B = A \cup B$
$\blacksquare$
Sources
- Mizar article YELLOW_1:9