Union (set theory)

The union of two sets A and B is the set of elements which are in A, in B, or in both A and B. In symbols,

${\displaystyle A\cup B=\{x:x\in A{\text{ or }}x\in B\}}$.[2]

For example, if A = {1, 3, 5, 7} and B = {1, 2, 4, 6, 7} then A ∪ B = {1, 2, 3, 4, 5, 6, 7}. A more elaborate example (involving two infinite sets) is:

A = {x is an even integer larger than 1}

B = {x is an odd integer larger than 1}

${\displaystyle A\cup B=\{2,3,4,5,6,\dots \}}$

As another example, 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.

Sets cannot have duplicate elements,[2][3] so the union of the sets {1, 2, 3} and {2, 3, 4} is {1, 2, 3, 4}. Multiple occurrences of identical elements have no effect on the cardinality of a set or its contents.

Binary union is an associative operation; that is, for any sets A, B, and C,

${\displaystyle A\cup (B\cup C)=(A\cup B)\cup C.}$

The operations can be performed in any order, and the parentheses may be omitted without ambiguity (i.e., either of the above can be expressed equivalently as A ∪ B ∪ C). Similarly, union is commutative, so the sets can be written in any order.[4]

The empty set is an identity element for the operation of union. That is, A ∪ ∅ = A, for any set A. This follows from analogous facts about logical disjunction.

Since sets with unions and intersections form a Boolean algebra, intersection distributes over union

${\displaystyle A\cap (B\cup C)=(A\cap B)\cup (A\cap C)}$

and union distributes over intersection

${\displaystyle A\cup (B\cap C)=(A\cup B)\cap (A\cup C)}$ .

Within a given universal set, union can be written in terms of the operations of intersection and complement as

${\displaystyle A\cup B=\left(A^{C}\cap B^{C}\right)^{C}}$

where the superscript ^C denotes the complement with respect to the universal set.

One can take the union of several sets simultaneously. For example, the union of three sets A, B, and C contains all elements of A, all elements of B, and all elements of C, and nothing else. Thus, x is an element of A ∪ B ∪ C if and only if x is in at least one of A, B, and C.

A finite union is the union of a finite number of sets; the phrase does not imply that the union set is a finite set.[5][6]

The most general notion is the union of an arbitrary collection of sets, sometimes called an infinitary union. If M is a set or class whose elements are sets, then x is an element of the union of M if and only if there is at least one element A of M such that x is an element of A.[7] In symbols:

${\displaystyle x\in \bigcup \mathbf {M} \iff \exists A\in \mathbf {M} ,\ x\in A.}$

This idea subsumes the preceding sections—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.

• Alternation (formal language theory), the union of sets of strings
• Disjoint union
• Intersection (set theory)
• Iterated binary operation
• Naive set theory
• Symmetric difference

Wikimedia Commons has media related to Union (set theory).

• Weisstein, Eric W. "Union". MathWorld.
• Hazewinkel, Michiel, ed. (2001) [1994], "Union of sets", Encyclopedia of Mathematics, Springer Science+Business Media B.V. / Kluwer Academic Publishers, ISBN 978-1-55608-010-4
• Infinite Union and Intersection at ProvenMath De Morgan's laws formally proven from the axioms of set theory.