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Union (set theory)

## Summary

In set theory, the union (denoted by ∪) of a collection of sets is the set of all elements in the collection.[1] It is one of the fundamental operations through which sets can be combined and related to each other. A nullary union refers to a union of zero (${\displaystyle 0}$) sets and it is by definition equal to the empty set.

Union of two sets:
${\displaystyle ~A\cup B}$
Union of three sets:
${\displaystyle ~A\cup B\cup C}$
The union of A, B, C, D, and E is everything except the white area.

For explanation of the symbols used in this article, refer to the table of mathematical symbols.

## Union of two sets

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.[2] In set-builder notation,

${\displaystyle A\cup B=\{x:x\in A{\text{ or }}x\in B\}}$ .[3]

For example, if A = {1, 3, 5, 7} and B = {1, 2, 4, 6, 7} then AB = {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,[3][4] 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.

## Algebraic properties

Binary union is an associative operation; that is, for any sets ${\displaystyle A,B,{\text{ and }}C,}$

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

Thus the parentheses may be omitted without ambiguity: either of the above can be written as ${\displaystyle A\cup B\cup C.}$  Also, union is commutative, so the sets can be written in any order.[5] The empty set is an identity element for the operation of union. That is, ${\displaystyle A\cup \varnothing =A,}$  for any set ${\displaystyle A.}$  Also, the union operation is idempotent: ${\displaystyle A\cup A=A.}$  All these properties follow from analogous facts about logical disjunction.

Intersection distributes over union

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

and union distributes over intersection[2]
${\displaystyle A\cup (B\cap C)=(A\cup B)\cap (A\cup C).}$

The power set of a set ${\displaystyle U,}$  together with the operations given by union, intersection, and complementation, is a Boolean algebra. In this Boolean algebra, union can be expressed in terms of intersection and complementation by the formula

${\displaystyle A\cup B=\left(A^{\text{c}}\cap B^{\text{c}}\right)^{\text{c}},}$

where the superscript ${\displaystyle {}^{\text{c}}}$  denotes the complement in the universal set ${\displaystyle U.}$

## Finite unions

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 ABC 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.[6][7]

## Arbitrary unions

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.[8] 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, ABC 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.

### Notations

The notation for the general concept can vary considerably. For a finite union of sets ${\displaystyle S_{1},S_{2},S_{3},\dots ,S_{n}}$  one often writes ${\displaystyle S_{1}\cup S_{2}\cup S_{3}\cup \dots \cup S_{n}}$  or ${\displaystyle \bigcup _{i=1}^{n}S_{i}}$ . Various common notations for arbitrary unions include ${\displaystyle \bigcup \mathbf {M} }$ , ${\displaystyle \bigcup _{A\in \mathbf {M} }A}$ , and ${\displaystyle \bigcup _{i\in I}A_{i}}$ . The last of these notations refers to the union of the collection ${\displaystyle \left\{A_{i}:i\in I\right\}}$ , where I is an index set and ${\displaystyle A_{i}}$  is a set for every ${\displaystyle i\in I}$ . In the case that the index set I is the set of natural numbers, one uses the notation ${\displaystyle \bigcup _{i=1}^{\infty }A_{i}}$ , which is analogous to that of the infinite sums in series.[8]

When the symbol "∪" is placed before other symbols (instead of between them), it is usually rendered as a larger size.

## Notation encoding

In Unicode, union is represented by the character U+222A UNION. In TeX, ${\displaystyle \cup }$  is rendered from \cup.