Table of contents |
2 Algebraic properties 3 Infinitary unions |
If A and B are sets, then the union of A and B is the set that contains all elements of A and all elements of B, but no other elements. The union of A and B is standardly written "A ∪ B". Formally:
For example, the union of the sets {1,2,3} and {2,3,4} is {1,2,3,4}. The number 9 is not contained in the union of the set of prime numbers {2,3,5,7,11,...} and the set of even numbers {2,4,6,8,10,...}, because 9 is neither prime nor even.
More generally, one can take the union of several sets at once. The union of A, B, and C, for example, contains all elements of A, all elements of B, and all elements of C, and nothing else. Formally, x is an element of A ∪ B ∪ C iff x is in A or x is in B or x is in C.
Binary union (the union of just two sets at a time) is an associative operation; that is, A ∪ (B ∪ C) = (A ∪ B) ∪ C. In fact, A ∪ B ∪ C is equal to both of these sets as well, so parentheses are never needed when writing only unions. Similarly, union is commutative, so you can write the sets in any order. The empty set is an identity element for the operation of union. That is, {} ∪ A = A, for any set A. Thus one can think of the empty set as the union of zero sets. In terms of the definitions, these facts follow from analogous facts about logical disjunction.
Together with intersection and complement, union makes any power set into a Boolean algebra. For example union and intersection distributes over each other, and all three operations are combined in de Morgan's laws. If you want a Boolean ring instead of a Boolean algebra, then you can replace union with symmetric difference.
The most general notion is the union of an arbitrary collection of sets. If M is a set whose elements are themselves sets, then x is an element of the union of M if and only if for at least one element A of M, x is an element of A. In symbols:
This idea subsumes the above paragraphs, in that for example, A ∪ B ∪ C is the union of the collection {A,B,C}. Also, if M is the empty collection, then the union of M is the empty set. The analogy between finitary unions and logical disjunction extends to one between infinitary unions and existential quantification.
The notation for the general concept can vary considerably. Hardcore set theorists will simply write
Intersection distributes over infinitary union, in the sense that