Axiom·A03
Union
The union of any family of sets is a set.
The set
is the union
of D004.
In words
For any family ℱ, there is a set U where for every x, x belongs to U exactly when there is some set A with A in the family and x in A.
This is a ZFC axiom: it is assumed, not proven. Everything below it in a proof chain ultimately rests here.
Remarks
Used by
Propose an edit3 published revisions