# Union (set theory)

In set theory, the union (denoted by ∪) of a collection of sets is the set of all elements in the collection. 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 (⁠$0$⁠) sets and it is by definition equal to the empty set.
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. In set-builder notation,
    $A\cup B=\{x:x\in A{\text{ or }}x\in B\}$.
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}
    $A\cup B=\{2,3,4,5,6,\dots \}$
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, 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 ⁠$A,B,{\text{ and }}C$⁠, $A\cup (B\cup C)=(A\cup B)\cup C.$ Thus, the parentheses may be omitted without ambiguity: either of the above can be written as ⁠$A\cup B\cup C$⁠. Also, union is commutative, so the sets can be written in any order. The empty set is an identity element for the operation of union. That is, ⁠$A\cup \varnothing =A$⁠, for any set ⁠$A$⁠. Also, the union operation is idempotent: ⁠$A\cup A=A$⁠. All these properties follow from analogous facts about logical disjunction.
Intersection distributes over union $A\cap (B\cup C)=(A\cap B)\cup (A\cap C)$ and union distributes over intersection $A\cup (B\cap C)=(A\cup B)\cap (A\cup C).$ The power set of a set ⁠$U$⁠, 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 $A\cup B=(A^{\complement }\cap B^{\complement })^{\complement },$ where the superscript $^{\complement }$ denotes the complement in the universal set ⁠$U$⁠. Alternatively, intersection can be expressed in terms of union and complementation in a similar way: $A\cap B=(A^{\complement }\cup B^{\complement })^{\complement }$. These two expressions together are called De Morgan's laws.

## 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 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.

## 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. In symbols:
    $x\in \bigcup \mathbf {M} \iff \exists A\in \mathbf {M} ,\ x\in A.$
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.

### Notations

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