In mathematics, a set is a collection of distinct objects, considered
as an object in its own right. For example, the numbers 2, 4, and 6
are distinct objects when considered separately, but when they are
considered collectively they form a single set of size three, written
2,4,6 . The concept of a set is one of the most fundamental in
mathematics. Developed at the end of the 19th century, set theory is
now a ubiquitous part of mathematics, and can be used as a foundation
from which nearly all of mathematics can be derived. In mathematics
education, elementary topics such as Venn diagrams are taught at a
young age, while more advanced concepts are taught as part of a
The German word Menge, rendered as "set" in English, was coined by
1 Definition 2 Describing sets 3 Membership
3.1 Subsets 3.2 Power sets
6.1 Unions 6.2 Intersections 6.3 Complements 6.4 Cartesian product
7 Applications 8 Axiomatic set theory 9 Principle of inclusion and exclusion 10 De Morgan's laws 11 See also 12 Notes 13 References 14 External links
Passage with a translation of the original set definition of Georg Cantor. The German word Menge for set is translated with aggregate here.
A set is a well-defined collection of distinct objects. The objects that make up a set (also known as the set's elements or members) can be anything: numbers, people, letters of the alphabet, other sets, and so on. Georg Cantor, one of the founders of set theory, gave the following definition of a set at the beginning of his Beiträge zur Begründung der transfiniten Mengenlehre:
A set is a gathering together into a whole of definite, distinct objects of our perception [Anschauung] or of our thought—which are called elements of the set.
Sets are conventionally denoted with capital letters. Sets A and B are equal if and only if they have precisely the same elements. For technical reasons, Cantor's definition turned out to be inadequate; today, in contexts where more rigor is required, one can use axiomatic set theory, in which the notion of a "set" is taken as a primitive notion and the properties of sets are defined by a collection of axioms. The most basic properties are that a set can have elements, and that two sets are equal (one and the same) if and only if every element of each set is an element of the other; this property is called the extensionality of sets. Describing sets Main article: Set notation There are two ways of describing, or specifying the members of, a set. One way is by intensional definition, using a rule or semantic description:
A is the set whose members are the first four positive integers. B is the set of colors of the French flag.
The second way is by extension – that is, listing each member of the set. An extensional definition is denoted by enclosing the list of members in curly brackets:
C = 4, 2, 1, 3 D = blue, white, red .
One often has the choice of specifying a set either intensionally or extensionally. In the examples above, for instance, A = C and B = D. In an extensional definition, a set member can be listed two or more times, for example, 11, 6, 6 . However, per extensionality, two definitions of sets which differ only in that one of the definitions lists set members multiple times, define, in fact, the same set. Hence, the set 11, 6, 6 is exactly identical to the set 11, 6 . Moreover, the order in which the elements of a set are listed is irrelevant (unlike for a sequence or tuple). We can illustrate these two important points with an example:
6, 11 = 11, 6 = 11, 6, 6, 11 .
For sets with many elements, the enumeration of members can be abbreviated. For instance, the set of the first thousand positive integers may be specified extensionally as
1, 2, 3, ..., 1000 ,
where the ellipsis ("...") indicates that the list continues in the obvious way. Ellipses may also be used where sets have infinitely many members. Thus the set of positive even numbers can be written as 2, 4, 6, 8, ... . The notation with braces may also be used in an intensional specification of a set. In this usage, the braces have the meaning "the set of all ...". So, E = playing card suits is the set whose four members are ♠, ♦, ♥, and ♣. A more general form of this is set-builder notation, through which, for instance, the set F of the twenty smallest integers that are four less than perfect square can be denoted
F = n2 − 4 : n is an integer; and 0 ≤ n ≤ 19 .
In this notation, the colon (":") means "such that", and the description can be interpreted as "F is the set of all numbers of the form n2 − 4, such that n is a whole number in the range from 0 to 19 inclusive." Sometimes the vertical bar ("") is used instead of the colon. Membership Main article: Element (mathematics) If B is a set and x is one of the objects of B, this is denoted x ∈ B, and is read as "x belongs to B", or "x is an element of B". If y is not a member of B then this is written as y ∉ B, and is read as "y does not belong to B". For example, with respect to the sets A = 1,2,3,4 , B = blue, white, red , and F = n2 − 4 : n is an integer; and 0 ≤ n ≤ 19 defined above,
4 ∈ A and 12 ∈ F; but 9 ∉ F and green ∉ B.
Subsets Main article: Subset If every member of set A is also a member of set B, then A is said to be a subset of B, written A ⊆ B (also pronounced A is contained in B). Equivalently, we can write B ⊇ A, read as B is a superset of A, B includes A, or B contains A. The relationship between sets established by ⊆ is called inclusion or containment. If A is a subset of, but not equal to, B, then A is called a proper subset of B, written A ⊊ B (A is a proper subset of B) or B ⊋ A (B is a proper superset of A). The expressions A ⊂ B and B ⊃ A are used differently by different authors; some authors use them to mean the same as A ⊆ B (respectively B ⊇ A), whereas others use them to mean the same as A ⊊ B (respectively B ⊋ A).
A is a subset of B
The set of all men is a proper subset of the set of all people. 1, 3 ⊆ 1, 2, 3, 4 . 1, 2, 3, 4 ⊆ 1, 2, 3, 4 .
The empty set is a subset of every set and every set is a subset of itself:
Every set is a subset of the universal set:
A ⊆ U.
An obvious but useful identity, which can often be used to show that two seemingly different sets are equal:
A = B if and only if A ⊆ B and B ⊆ A.
A partition of a set S is a set of nonempty subsets of S such that
every element x in S is in exactly one of these subsets.
Main article: Power set
The power set of a set S is the set of all subsets of S. The power set
contains S itself and the empty set because these are both subsets of
S. For example, the power set of the set 1, 2, 3 is 1, 2, 3 , 1,
2 , 1, 3 , 2, 3 , 1 , 2 , 3 ,
P or ℙ, denoting the set of all primes: P = 2, 3, 5, 7, 11, 13, 17, ... . N or
displaystyle mathbb N
, denoting the set of all natural numbers: N = 0, 1, 2, 3, . . . (sometimes defined excluding 0). Z or
displaystyle mathbb Z
, denoting the set of all integers (whether positive, negative or zero): Z = ..., −2, −1, 0, 1, 2, ... . Q or ℚ, denoting the set of all rational numbers (that is, the set of all proper and improper fractions): Q = a/b : a, b ∈ Z, b ≠ 0 . For example, 1/4 ∈ Q and 11/6 ∈ Q. All integers are in this set since every integer a can be expressed as the fraction a/1 (Z ⊊ Q). R or
displaystyle mathbb R
, denoting the set of all real numbers. This set includes all rational numbers, together with all irrational numbers (that is, algebraic numbers that cannot be rewritten as fractions such as √2, as well as transcendental numbers such as π, e). C or ℂ, denoting the set of all complex numbers: C = a + bi : a, b ∈ R . For example, 1 + 2i ∈ C. H or ℍ, denoting the set of all quaternions: H = a + bi + cj + dk : a, b, c, d ∈ R . For example, 1 + i + 2j − k ∈ H.
Positive and negative sets are denoted by a superscript - or +. For example, ℚ+ represents the set of positive rational numbers. Each of the above sets of numbers has an infinite number of elements, and each can be considered to be a proper subset of the sets listed below it. The primes are used less frequently than the others outside of number theory and related fields. Basic operations There are several fundamental operations for constructing new sets from given sets. Unions
The union of A and B, denoted A ∪ B
Main article: Union (set theory) Two sets can be "added" together. The union of A and B, denoted by A ∪ B, is the set of all things that are members of either A or B. Examples:
1, 2 ∪ 1, 2 = 1, 2 . 1, 2 ∪ 2, 3 = 1, 2, 3 . 1, 2, 3 ∪ 3, 4, 5 = 1, 2, 3, 4, 5
Some basic properties of unions:
A ∪ B = B ∪ A.
A ∪ (B ∪ C) = (A ∪ B) ∪ C.
A ⊆ (A ∪ B).
A ∪ A = A.
A ∪ U = U.
Intersections Main article: Intersection (set theory) A new set can also be constructed by determining which members two sets have "in common". The intersection of A and B, denoted by A ∩ B, is the set of all things that are members of both A and B. If A ∩ B = ∅, then A and B are said to be disjoint.
The intersection of A and B, denoted A ∩ B.
1, 2 ∩ 1, 2 = 1, 2 . 1, 2 ∩ 2, 3 = 2 .
Some basic properties of intersections:
A ∩ B = B ∩ A.
A ∩ (B ∩ C) = (A ∩ B) ∩ C.
A ∩ B ⊆ A.
A ∩ A = A.
A ∩ U = A.
The relative complement of B in A
The complement of A in U
The symmetric difference of A and B
Main article: Complement (set theory) Two sets can also be "subtracted". The relative complement of B in A (also called the set-theoretic difference of A and B), denoted by A B (or A − B), is the set of all elements that are members of A but not members of B. Note that it is valid to "subtract" members of a set that are not in the set, such as removing the element green from the set 1, 2, 3 ; doing so has no effect. In certain settings all sets under discussion are considered to be subsets of a given universal set U. In such cases, U A is called the absolute complement or simply complement of A, and is denoted by A′.
A′ = U A
1, 2 1, 2 = ∅. 1, 2, 3, 4 1, 3 = 2, 4 . If U is the set of integers, E is the set of even integers, and O is the set of odd integers, then U E = E′ = O.
Some basic properties of complements:
An extension of the complement is the symmetric difference, defined for sets A, B as
B = ( A ∖ B ) ∪ ( B ∖ A ) .
displaystyle A,Delta ,B=(Asetminus B)cup (Bsetminus A).
For example, the symmetric difference of 7,8,9,10 and 9,10,11,12
is the set 7,8,11,12 . The power set of any set becomes a Boolean
ring with symmetric difference as the addition of the ring (with the
empty set as neutral element) and intersection as the multiplication
of the ring.
Main article: Cartesian product
A new set can be constructed by associating every element of one set
with every element of another set. The
1, 2 × red, white, green = (1, red), (1, white), (1, green), (2, red), (2, white), (2, green) . 1, 2 × 1, 2 = (1, 1), (1, 2), (2, 1), (2, 2) . a, b, c × d, e, f = (a, d), (a, e), (a, f), (b, d), (b, e), (b, f), (c, d), (c, e), (c, f) .
Some basic properties of Cartesian products:
A × B ≠ B × A for A ≠ B.
Let A and B be finite sets; then the cardinality of the Cartesian product is the product of the cardinalities:
A × B = B × A = A × B .
Russell's paradox—It shows that the "set of all sets that do not contain themselves," i.e. the "set" x : x is a set and x ∉ x does not exist. Cantor's paradox—It shows that "the set of all sets" cannot exist.
The reason is that the phrase well-defined is not very well-defined. It was important to free set theory of these paradoxes because nearly all of mathematics was being redefined in terms of set theory. In an attempt to avoid these paradoxes, set theory was axiomatized based on first-order logic, and thus axiomatic set theory was born. For most purposes, however, naive set theory is still useful. Principle of inclusion and exclusion Main article: Inclusion–exclusion principle
The inclusion-exclusion principle can be used to calculate the size of the union of sets: the size of the union is the size of the two sets, minus the size of their intersection.
The inclusion–exclusion principle is a counting technique that can be used to count the number of elements in a union of two sets, if the size of each set and the size of their intersection are known. It can be expressed symbolically as
A ∪ B
A ∩ B
displaystyle Acup B=A+B-Acap B.
A more general form of the principle can be used to find the cardinality of any finite union of sets:
∪ … ∪
n − 1
n − 1
∩ … ∩
displaystyle begin aligned leftA_ 1 cup A_ 2 cup A_ 3 cup ldots cup A_ n right=&left(leftA_ 1 right+leftA_ 2 right+leftA_ 3 right+ldots leftA_ n rightright)\& -left(leftA_ 1 cap A_ 2 right+leftA_ 1 cap A_ 3 right+ldots leftA_ n-1 cap A_ n rightright)\& +ldots \& +left(-1right)^ n-1 left(leftA_ 1 cap A_ 2 cap A_ 3 cap ldots cap A_ n rightright).end aligned
De Morgan's laws
Augustus De Morgan
(A ∪ B)′ = A′ ∩ B′
The complement of A union B equals the complement of A intersected with the complement of B.
(A ∩ B)′ = A′ ∪ B′
The complement of A intersected with B is equal to the complement of A union to the complement of B. See also
Alternative set theory
Axiomatic set theory
Category of sets
Class (set theory)
Family of sets
Naive set theory
^ "Eine Menge, ist die Zusammenfassung bestimmter, wohlunterschiedener Objekte unserer Anschauung oder unseres Denkens – welche Elemente der Menge genannt werden – zu einem Ganzen." "Archived copy". Archived from the original on 2011-06-10. Retrieved 2011-04-22. ^ a b Stoll, Robert. Sets, Logic and Axiomatic Theories. W. H. Freeman and Company. p. 5.
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