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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 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 In set theory and its applications to logic, mathematics, and computer science, set-builder notation is a mathematical notation for describing a set by enumerating its elements, or stating the properties that its members must satisfy. Defining ...
, :A \cup B = \. For example, if ''A'' = and ''B'' = then ''A'' ∪ ''B'' = . A more elaborate example (involving two infinite sets) is: : ''A'' = : ''B'' = : A \cup B = \ As another example, the number 9 is ''not'' contained in the union of the set of
prime number A prime number (or a prime) is a natural number greater than 1 that is not a product of two smaller natural numbers. A natural number greater than 1 that is not prime is called a composite number. For example, 5 is prime because the only way ...
s 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, \text 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 In mathematics, an identity element, or neutral element, of a binary operation operating on a set is an element of the set that leaves unchanged every element of the set when the operation is applied. This concept is used in algebraic structures s ...
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 mathematics and mathematical logic, Boolean algebra is a branch of algebra. It differs from elementary algebra in two ways. First, the values of the variables are the truth values ''true'' and ''false'', usually denoted 1 and 0, whereas ...
. In this Boolean algebra, union can be expressed in terms of intersection and complementation by the formula A \cup B = \left(A^\text \cap B^\text \right)^\text, where the superscript ^\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 In mathematics, particularly set theory, a finite set is a set that has a finite number of elements. Informally, a finite set is a set which one could in principle count and finish counting. For example, :\ is a finite set with five elements. ...
.


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 \iff \exists A \in \mathbf,\ x \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, \dots , S_n one often writes S_1 \cup S_2 \cup S_3 \cup \dots \cup S_n or \bigcup_^n S_i. Various common notations for arbitrary unions include \bigcup \mathbf, \bigcup_ A, and \bigcup_ A_. The last of these notations refers to the union of the collection \left\, 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 number In mathematics, the natural numbers are those numbers used for counting (as in "there are ''six'' coins on the table") and ordering (as in "this is the ''third'' largest city in the country"). Numbers used for counting are called '' cardinal ...
s, one uses the notation \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, \cup is rendered from \cup.


See also

* * − the union of sets of strings * * * * * * * *


Notes


External links

*
Infinite Union and Intersection at ProvenMath
De Morgan's laws formally proven from the axioms of set theory. {{Improve categories, date=May 2021 Boolean algebra Basic concepts in set theory Operations on sets Set theory