Disjoint union


In mathematics, the disjoint union (or discriminated union) of the sets A and B is the set formed from the elements of A and B labelled (indexed) with the name of the set from which they come. So, an element belonging to both A and B appears twice in the disjoint union, with two different labels.

Disjoint union
TypeSet operation
FieldSet theory
Symbolic statement

A disjoint union of an indexed family of sets is a set often denoted by with an injection of each into such that the images of these injections form a partition of (that is, each element of belongs to exactly one of these images). A disjoint union of a family of pairwise disjoint sets is their union.

In category theory, the disjoint union is the coproduct of the category of sets, and thus defined up to a bijection. In this context, the notation is often used.

The disjoint union of two sets and is written with infix notation as . Some authors use the alternative notation or (along with the corresponding or ).

A standard way for building the disjoint union is to define as the set of ordered pairs such that and the injection as

Example edit

Consider the sets   and   It is possible to index the set elements according to set origin by forming the associated sets  

where the second element in each pair matches the subscript of the origin set (for example, the   in   matches the subscript in   etc.). The disjoint union   can then be calculated as follows:


Set theory definition edit

Formally, let   be an indexed family of sets indexed by   The disjoint union of this family is the set

The elements of the disjoint union are ordered pairs   Here   serves as an auxiliary index that indicates which   the element   came from.

Each of the sets   is canonically isomorphic to the set

Through this isomorphism, one may consider that   is canonically embedded in the disjoint union. For   the sets   and   are disjoint even if the sets   and   are not.

In the extreme case where each of the   is equal to some fixed set   for each   the disjoint union is the Cartesian product of   and  :


Occasionally, the notation

is used for the disjoint union of a family of sets, or the notation   for the disjoint union of two sets. This notation is meant to be suggestive of the fact that the cardinality of the disjoint union is the sum of the cardinalities of the terms in the family. Compare this to the notation for the Cartesian product of a family of sets.

In the language of category theory, the disjoint union is the coproduct in the category of sets. It therefore satisfies the associated universal property. This also means that the disjoint union is the categorical dual of the Cartesian product construction. See Coproduct for more details.

For many purposes, the particular choice of auxiliary index is unimportant, and in a simplifying abuse of notation, the indexed family can be treated simply as a collection of sets. In this case   is referred to as a copy of   and the notation   is sometimes used.

Category theory point of view edit

In category theory the disjoint union is defined as a coproduct in the category of sets.

As such, the disjoint union is defined up to an isomorphism, and the above definition is just one realization of the coproduct, among others. When the sets are pairwise disjoint, the usual union is another realization of the coproduct. This justifies the second definition in the lead.

This categorical aspect of the disjoint union explains why   is frequently used, instead of   to denote coproduct.

See also edit

  • Coproduct – Category-theoretic construction
  • Direct limit – Special case of colimit in category theory
  • Disjoint union (topology) – space formed by equipping the disjoint union of the underlying sets with a natural topology called the disjoint union topology
  • Disjoint union of graphs – Combining the vertex and edge sets of two graphs
  • Intersection (set theory) – Set of elements common to all of some sets
  • List of set identities and relations – Equalities for combinations of sets
  • Partition of a set – Mathematical ways to group elements of a set
  • Sum type – Data structure used to hold a value that could take on several different, but fixed, types
  • Symmetric difference – Elements in exactly one of two sets
  • Tagged union – Data structure used to hold a value that could take on several different, but fixed, types
  • Union (computer science) – Variable able to hold different data types

References edit

  • Lang, Serge (2004), Algebra, Graduate Texts in Mathematics, vol. 211 (Corrected fourth printing, revised third ed.), New York: Springer-Verlag, p. 60, ISBN 978-0-387-95385-4
  • Weisstein, Eric W. "Disjoint Union". MathWorld.