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.
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 symbols,
:$A\; \backslash cup\; B\; =\; \backslash $.
For example, if ''A'' = and ''B'' = then ''A'' ∪ ''B'' = . A more elaborate example (involving two infinite sets) is:
: ''A'' =
: ''B'' =
: $A\; \backslash cup\; B\; =\; \backslash $
As another example, the number 9 is ''not'' contained in the union of the set of prime numbers and the set of even numbers , because 9 is neither prime nor even.
Sets cannot have duplicate elements, so the union of the sets and is . 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'', and ''C'',
:$A\; \backslash cup\; (B\; \backslash cup\; C)\; =\; (A\; \backslash cup\; B)\; \backslash cup\; C.$
Thus the parentheses may be omitted without ambiguity: either of the above can be written as ''A'' ∪ ''B'' ∪ ''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'' ∪ ∅ = ''A'', for any set ''A.'' Also, the union operation is idempotent: ''A'' ∪ ''A'' = ''A''. All these properties follow from analogous facts about logical disjunction.
Intersection distributes over union
:$A\; \backslash cap\; (B\; \backslash cup\; C)\; =\; (A\; \backslash cap\; B)\backslash cup(A\; \backslash cap\; C)$
and union distributes over intersection
:$A\; \backslash cup\; (B\; \backslash cap\; C)\; =\; (A\; \backslash cup\; B)\; \backslash cap\; (A\; \backslash cup\; C).$
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
:$A\; \backslash cup\; B\; =\; \backslash left(A^\backslash text\; \backslash cap\; B^\backslash text\; \backslash right)^\backslash text,$
where the superscript $^\backslash text$ denotes the complement in the universal set ''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 ''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:
: $x\; \backslash in\; \backslash bigcup\; \backslash mathbf\; \backslash iff\; \backslash exists\; A\; \backslash in\; \backslash mathbf,\backslash \; x\; \backslash in\; A.$
This idea subsumes the preceding sections—for example, ''A'' ∪ ''B'' ∪ ''C'' is the union of the collection . 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 $S\_1,\; S\_2,\; S\_3,\; \backslash dots\; ,\; S\_n$ one often writes $S\_1\; \backslash cup\; S\_2\; \backslash cup\; S\_3\; \backslash cup\; \backslash dots\; \backslash cup\; S\_n$ or $\backslash bigcup\_^n\; S\_i$. Various common notations for arbitrary unions include $\backslash bigcup\; \backslash mathbf$, $\backslash bigcup\_\; A$, and $\backslash bigcup\_\; A\_$. The last of these notations refers to the union of the collection $\backslash left\backslash $, where ''I'' is an index set and $A\_i$ is a set for every $i\; \backslash in\; I$. In the case that the index set ''I'' is the set of natural numbers, one uses the notation $\backslash bigcup\_^\; A\_$, 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 . In TeX, $\backslash cup$ is rendered from \cup.

** See also **

* Algebra of sets
* Alternation (formal language theory), the union of sets of strings
* Axiom of union
* Disjoint union
* Intersection (set theory)
* Iterated binary operation
* List of set identities and relations
* Naive set theory
* Symmetric difference

Notes

** External links **

*

Infinite Union and Intersection at ProvenMath

De Morgan's laws formally proven from the axioms of set theory. {{Set theory Category:Operations on sets

Notation encoding

In Unicode, union is represented by the character . In TeX, $\backslash cup$ is rendered from \cup.

Notes

Infinite Union and Intersection at ProvenMath

De Morgan's laws formally proven from the axioms of set theory. {{Set theory Category:Operations on sets