# Complement (set theory)

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Complement (set theory)

In set theory, a complement of a set A refers to things not in (that is, things outside of), A. The relative complement of A with respect to a set B, is the set of elements in B but not in A. When all sets under consideration are considered to be subsets of a given set U, the absolute complement of A is the set of all elements in U but not in A.

## 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 (left circle) in B (right circle):
$A^c \cap B~~~~=~~~~B \setminus A$

The relative complement of A in B is denoted B ∖ A according to the ISO 31-11 standard (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

$B \setminus A = \{ x\in B \, | \, x \notin A \}.$

Examples:

• {1,2,3} ∖ {2,3,4}   =   {1}
• {2,3,4} ∖ {1,2,3}   =   {4}
• If $\mathbb{R}$ is the set of real numbers and $\mathbb{Q}$ is the set of rational numbers, then $\mathbb{R}\setminus\mathbb{Q} = \mathbb{J}$ is the set of irrational numbers.

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

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

## Absolute complement

The complement of A in U:
$A^c=U \setminus A$

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 integers, then the complement of the set of odd numbers is the set of even numbers.

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

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

De Morgan's laws:
• $\left(A \cup B \right)^{c}=A^{c} \cap B^{c} .$
• $\left(A \cap B \right)^{c}=A^{c} \cup B^{c} .$
Complement laws:
• $A \cup A^{c} =U .$
• $A \cap A^{c} =\empty .$
• $\empty ^{c} =U.$
• $U^{c} =\empty.$
• $\text{If }A\subset B\text{, then }B^{c}\subset A^{c}.$
(this follows from the equivalence of a conditional with its contrapositive)
Involution or double complement law:
• $\left(A^{c}\right)^{c}=A.$
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, proper subset of U, then {A, Ac} is a partition of U.

## 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{\backslash}. A variant \smallsetminus is available in the amssymb package.

## Complements in various programming languages

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:

SQL
SELECT * FROM B LEFT OUTER JOIN A ON B.COLUMN = A.COLUMN WHERE A.ID IS NULL
Mathematica
Complement[1]
MATLAB
setdiff[2]
MathML
<apply xmlns="http://www.w3.org/1998/Math/MathML"> <setdiff/> <ci type="set">A</ci> <ci type="set">B</ci></apply>
Pascal
SetDifference := a - b;
Python
diff = a.difference(b)[3]
diff = a - b[3]
Java
diff = a.clone();
diff.removeAll(b);[4]
Scala
diff = a -- b[5]
C++
set_difference(a.begin(), a.end(), b.begin(), b.end(), result.begin());
.NET Framework
a.Except(b);
a \\ b [6]
Common Lisp
set-difference, nset-difference[7]
OCaml
Set.S.diff[8]
Unix shell
comm -23 a b[9]
PHP
array_diff($a,$b);[10]
R
setdiff[11]
Ruby
diff = a - b[12]
Perl
#for perl version >= 5.10
@a = grep {not \$_ ~~ @b} @a;

## References

1. ^ Complement. Mathematica Documentation Center for version 6.0, updated in 2008. Accessed on March 7, 2008.
2. ^ Setdiff. MATLAB Function Reference for version 7.6, updated in 2008. Accessed on May 19, 2008.
3. ^ a b Set Types -- set, frozenset. Python Library Reference release 2.5, updated on September 19, 2006. Accessed on February 13, 2008.
4. ^ Set (Java 2 Platform SE 5.0). JavaTM 2 Platform Standard Edition 5.0 API Specification, updated in 2004. Accessed on February 13, 2008.
5. ^ scala.collection.Set. Scala Standard Library release 2.8.1, Accessed on December 09, 2010.
7. ^ Common Lisp HyperSpec, Function set-difference, nset-difference. Accessed on September 8, 2009.
8. ^ Set.S (OCaml).
9. ^ comm(1), Unix Seventh Edition Manual.
10. ^ PHP: array_diff, PHP Manual
11. ^ R Reference manual p. 410.
12. ^ Class: Array Ruby Documentation

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