Union (set theory)
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.
) 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.
A more elaborate example (involving two infinite sets) is: 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.
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
Thus, the parentheses may be omitted without ambiguity: either of the above can be written as
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.
Also, the union operation is idempotent:
All these properties follow from analogous facts about logical disjunction.
Intersection distributes over union
and union distributes over intersection[2]
, 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
denotes the complement in the universal set
Alternatively, intersection can be expressed in terms of union and complementation in a similar way:
These two expressions together are called De Morgan's laws.
[6][7][8] One can take the union of several sets simultaneously.
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.
[9][10] The most general notion is the union of an arbitrary collection of sets, sometimes called an infinitary union.
[11] In symbols: This idea subsumes the preceding sections—for example, A ∪ B ∪ C is the union of the collection {A, B, C}.
The notation for the general concept can vary considerably.
For a finite union of sets
Various common notations for arbitrary unions include
The last of these notations refers to the union of the collection
In the case that the index set I is the set of natural numbers, one uses the notation
, which is analogous to that of the infinite sums in series.
[11] When the symbol "∪" is placed before other symbols (instead of between them), it is usually rendered as a larger size.
