Union

Set

context $X,Y$ … set
definiendum $ x\in X \cup Y $
postulate $ x\in X\lor x\in Y $

Discussion

The 2-ary set construction $ X \cup Y $ is commutative and idempotent.

Notice that $ X \cup \emptyset = X$.

The union and intersection are associative and distributive with respect to another.

Reference

Wikipedia: Union

Parents

Context

Sets