Set System Closed under Union is Commutative Semigroup
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Theorem
Let $\SS$ be a system of sets.
Let $\SS$ be such that:
- $\forall A, B \in \SS: A \cup B \in \SS$
Then $\struct {\SS, \cup}$ is a commutative semigroup.
Proof
Closure
We have by hypothesis that $\struct {\SS, \cup}$ is closed.
Associativity
The operation $\cup$ is associative from Union is Associative.
Commutativity
The operation $\cup$ is commutative from Union is Commutative.
Hence, by definition, the result.
$\blacksquare$