# Set System Closed under Union is Commutative Semigroup

## 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.

$\blacksquare$