Complement (set theory)


In set theory, the complement of a set, often denoted by , is the set of elements not in.
When all elements in the universe, i.e. all elements under consideration, are considered to be members of a given set, the absolute complement of is the set of elements in that are not in.
The relative complement of with respect to a set, also termed the set difference of and, written is the set of elements in that are not in.

Absolute complement

Definition

If is a set, then the absolute complement of is the set of elements not in . In other words, let be a set that contains all the elements under study; if there is no need to mention, either because it has been previously specified, or it is obvious and unique, then the absolute complement of is the relative complement of in :
The absolute complement of is usually denoted by. Other notations include

Examples

  • Assume that the universe is the set of integers. If is the set of odd numbers, then the complement of is the set of even numbers. If is the set of multiples of 3, then the complement of is the set of numbers congruent to 1 or 2 modulo 3.
  • Assume that the universe is the standard 52-card deck. If the set is the suit of spades, then the complement of is the union of the suits of clubs, diamonds, and hearts. If the set is the union of the suits of clubs and diamonds, then the complement of is the union of the suits of hearts and spades.
  • When the universe is the universe of sets described in formalized set theory, the absolute complement of a set is generally not itself a set, but rather a proper class. For more info, see universal set.

    Properties

Let and be two sets in a universe. The following identities capture important properties of absolute complements:
De Morgan's laws:
Complement laws:
  • :.
Involution or double complement law:
Relationships between relative and absolute complements:
Relationship with a set difference:
The first two complement laws above show that if is a non-empty, proper subset of, then is a partition of.

Relative complement

Definition

If and are sets, then the relative complement of in, also termed the set difference of and, is the set of elements in but not in.
The relative complement of in is denoted according to the ISO 31-11 standard. It is sometimes written but this notation is ambiguous, as in some contexts it can be interpreted as the set of all elements where is taken from and from.
Formally:

Examples

Let,, and be three sets in a universe. The following identities capture notable properties of relative complements:

Complementary relation

A binary relation is defined as a subset of a product of sets The complementary relation is the set complement of in The complement of relation can be written
Here, is often viewed as a logical matrix with rows representing the elements of and columns elements of The truth of corresponds to 1 in row column Producing the complementary relation to then corresponds to switching all 1s to 0s, and 0s to 1s for the logical matrix of the complement.
Together with composition of relations and converse relations, complementary relations and the algebra of sets are the elementary operations of the calculus of relations.

LaTeX notation

In the LaTeX typesetting language, the command \setminus is usually used for rendering a set difference symbol, which is similar to a backslash symbol. When rendered, the \setminus command looks identical to \backslash, except that it has a little more space in front and behind the slash, akin to the LaTeX sequence \mathbin. A variant \smallsetminus is available in the amssymb package, but this symbol is not included separately in Unicode. The symbol is produced by \complement.