Definitions

# Complement (set theory)

In discrete mathematics and predominantly in set theory, a complement is a concept used in comparisons of sets to refer to the unique values of one set in relation to another. The terms "absolute" and "relative" complement refer to more specific applications of the concept, with universal complements referring to elements unique to the universal set and the latter referring to the unique elements of one set in relation to another.

## Forms

### Absolute complement

If a universe U is defined, then the relative complement of A in U is called the absolute complement (or simply complement) of A, and is denoted by AC or sometimes A′, also the same set often is denoted by $complement_U A$ or $complement A$ if U is fixed), that is:

AC  = U  A.

For example, if the universe is the set of natural numbers, then the complement of the set of odd numbers is the set of even numbers.

The following proposition lists some important properties of absolute complements in relation to the set-theoretic operations of union and intersection.

PROPOSITION 2: If A and B are subsets of a universe U, then the following identities hold:

De Morgan's laws:
*(A ∪ B)C  = AC ∩ BC
*(A ∩ B)C  = AC ∪ BC
Complement laws:
*A ∪ AC  =  U
*A ∩ AC  =  Ø
C  =  U
*UC  =  Ø
*If AB, then BCAC (this follows from the equivalence of a conditional with its contrapositive)
Involution or double complement law:
*ACC  =  A.
Relationships between relative and absolute complements:
*A  B = A ∩ BC
*(A  B)C = AC ∪ B

The first two complement laws above shows that if A is a non-empty subset of U, then {A, AC} is a partition of U.

### Relative complement

If A and B are sets, then the relative complement of A in B, also known as the set-theoretic difference of B and A, is the set of elements in B, but not in A.

The relative complement of A in B is denoted B  A (sometimes written B − A, but this notation is ambiguous, as in some contexts it can be interpreted as the set of all b − a, where b is taken from B and a from A).

Formally:

$B setminus A = \left\{ xin B , | , x notin A \right\}.$

Examples:

*{1,2,3}  {2,3,4}   =   {1}
*{2,3,4}  {1,2,3}   =   {4}
*If $mathbb\left\{R\right\}$ is the set of real numbers and $mathbb\left\{Q\right\}$ is the set of rational numbers, then $mathbb\left\{R\right\}setminusmathbb\left\{Q\right\}$ is the set of irrational numbers.

The following proposition lists some notable properties of relative complements in relation to the set-theoretic operations of union and intersection.

PROPOSITION 1: If A, B, and C are sets, then the following identities hold:

*C  (A ∩ B)  =  (C  A)∪(C  B)
*C  (A ∪ B)  =  (C  A)∩(C  B)
*C  (B  A)  =  (A ∩ C)∪(C  B)
*(B  A) ∩ C  =  (B ∩ C)  A  =  B∩(C  A)
*(B  A) ∪ C  =  (B ∪ C)  (A  C)
*A  A  =  Ø
*Ø  A  =  Ø
*A  Ø  =  A

## Other applications

In the LaTeX typesetting language the command `setminus` is usually used for rendering a set difference symbol – a backslash-like 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{backslash}` . A variant `smallsetminus` is available in the amssymb package.

Some programming languages allow for manipulation of sets as data structures, using these operators or functions to construct the difference of sets `a` and `b`:Mathematica

`Complement`MATLAB
`setdiff`Python
`diff = a.difference(b)`
`diff = a - b`Java
`diff = a.clone(); diff.removeAll(b);`