In mathematics, a set is a well-defined collection of distinct elements or members.[1][2][3] The elements that make up a set can be anything: people, letters of the alphabet, or mathematical objects, such as numbers, points in space, lines or other geometrical shapes, algebraic constants and variables, or other sets.[4] Two sets are equal if and only if they have precisely the same elements.[5]
Sets are ubiquitous in modern mathematics. The more specialized subject of set theory is part of the foundations of mathematics, from which nearly all of mathematics can be derived.[6]
The concept of a set emerged in mathematics at the end of the 19th century.[7] The German word Menge, rendered as "set" in English, was coined by Bernard Bolzano in his work The Paradoxes of the Infinite.[8][9][10]
Georg Cantor was one of the founders of set theory. He gave the following definition of a set at the beginning of his Beiträge zur Begründung der transfiniten Mengenlehre:[11]
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.
A loose definition of sets, that any property without restriction may be used to form a set, leads to paradoxes. Axiomatic set theory takes the concept of a "set" as a primitive notion, and the properties of sets are defined by axioms.[12] 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.[13]
The standard mathematical notation for a finite set places its elements between a pair of curly braces. For example, the numbers 2, 4, and 6 are distinct objects when considered individually. When considered collectively, they form a single set of size three, written as {2, 4, 6}. The order in which the elements are listed in the set does not matter: in the example, this same set could also be written as {2, 6, 4}, {4, 2, 6}, {4, 6, 2}, {6, 2, 4} or {6, 4, 2}.[14]
Mathematical texts commonly use capital letters in italic such as A, B, C to denote sets.[15][16] Sets are conventionally denoted with capital letters.[17][18][15]
There are two common ways of describing or specifying the members of a set: roster notation and set builder notation.[19][20] These are examples of extensional and intensional definitions of sets, respectively.[21]
The Roster notation (or enumeration notation) method of defining a set consists of listing each member of the set.[19][22][23] More specifically, in roster notation (an example of extensional definition),[21] the set is denoted by enclosing the list of members in curly brackets:
For sets with many elements, the enumeration of members can be abbreviated.[24][25] For instance, the set of the first thousand positive integers may be specified in roster notation as
where the ellipsis ("...") indicates that the list continues according to the demonstrated pattern.[24]
In roster notation, listing a member repeatedly does not change the set, for example, the set {11, 6, 6} is identical to the set {11, 6}.[26][failed verification] Moreover, the order in which the elements of a set are listed is irrelevant (unlike for a sequence or tuple), so {6, 11} is yet again the same set.[26][16]
In set-builder notation, the set is specified as a selection from a larger set, determined by a condition involving the elements.[27][28] For example, a set F can be specified as follows:
In this notation, the vertical bar ("|") means "such that", and the description can be interpreted as "F is the set of all numbers n, such that n is an integer in the range from 0 to 19 inclusive". Sometimes, the colon (":") is used instead of the vertical bar.[29]
Set-builder notation is an example of intensional definition.[21]
Another method of defining a set is by using a rule or semantic description:[30]
This is another example of intensional definition.[21]
If B is a set and x is one of the objects of B, this is denoted as x ? B, and is read as "x is an element of B", as "x belongs to B", or "x is in B".[31] If y is not a member of B then this is written as y ? B, read as "y is not an element of B", or "y is not in B".[32][15][33]
For example, with respect to the sets A = {1, 2, 3, 4}, B = {blue, white, red}, and F = {n | n is an integer, and 0 n
If every element of set A is also in B, then A is said to be a subset of B, written A ? B (pronounced A is contained in B).[34] Equivalently, one can write B ? A, read as B is a superset of A, B includes A, or B contains A.[35][15] The relationship between sets established by ? is called inclusion or containment. Two sets are equal if they contain each other: A ? B and B ? A is equivalent to A = B.[27]
If A is a subset of B, but not equal to B, then A is called a proper subset of B, written A ? B, or simply A ? B[34] (A is a proper subset of B), or B ? A (B is a proper superset of A, B ? A).[15]
The expressions A ? B and B ? A are used differently by different authors; some authors use them to mean the same as A ? B[36][32] (respectively B ? A), whereas others use them to mean the same as A ? B[34] (respectively B ? A).
Examples:
There is a unique set with no members,[37] called the empty set (or the null set), which is denoted by the symbol ? or {} (other notations are used; see empty set).[15] The empty set is a subset of every set,[38] and every set is a subset of itself:[39]
An Euler diagram is a graphical representation of a set as a closed loop, enclosing its elements, or the relationships between different sets, as closed loops. If two sets have no members in common, the loops do not overlap.
This is distinct from a Venn diagram, which shows all possible relations between two or more sets, with each loop overlapping the others.
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. That is, the subsets are pairwise disjoint (meaning any two sets of the partition contain no element in common), and the union of all the subsets of the partition is S.[40][41]
The power set of a set S is the set of all subsets of S.[27] 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}, ?}. The power set of a set S is usually written as P(S).[27][42][15][16]
The power set of a finite set with n elements has 2n elements.[43] For example, the set {1, 2, 3} contains three elements, and the power set shown above contains 23 = 8 elements.
The power set of an infinite (either countable or uncountable) set is always uncountable. Moreover, the power set of a set is always strictly "bigger" than the original set, in the sense that there is no way to pair every element of S with exactly one element of P(S). (There is never an onto map or surjection from S onto P(S).)[44]
The cardinality of a set S, denoted ||, is the number of members of S.[45] For example, if B = {blue, white, red}, then . Repeated members in roster notation are not counted,[46][47] so , too.
The cardinality of the empty set is zero.[48]
Some sets have infinite cardinality. The set N of natural numbers, for instance, is infinite.[27] Some infinite cardinalities are greater than others. For instance, the set of real numbers has greater cardinality than the set of natural numbers.[49] However, it can be shown that the cardinality of a straight line (i.e., the number of points on a line) is the same as the cardinality of any segment of that line, of the entire plane, and indeed of any finite-dimensional Euclidean space.[50]
There are some sets or kinds of sets that hold great mathematical importance, and are referred to with such regularity that they have acquired special names--and notational conventions to identify them. One of these is the empty set, denoted { } or ?.[51][15] A set with exactly one element, x, is a unit set, or singleton, {x};[5] the latter is usually distinct from x.[52]
Many of these sets are represented using bold (e.g. P) or blackboard bold (e.g. P) typeface.[53] These include:[15]
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.
Positive and negative sets are sometimes denoted by superscript plus and minus signs, respectively. For example, Q+ represents the set of positive rational numbers.
There are several fundamental operations for constructing new sets from given sets.
Two sets can be "added" together. The union of A and B, denoted by A ? B,[15] is the set of all things that are members of either A or B.
Examples:
Some basic properties of unions:
A new set can also be constructed by determining which members two sets have "in common". The intersection of A and B, denoted by [15] is the set of all things that are members of both A and B. If then A and B are said to be disjoint.
Examples:
Some basic properties of intersections:
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 (or ),[15] is the set of all elements that are members of A, but not members of B. 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 will not affect the elements in the set.
In certain settings, all sets under discussion are considered to be subsets of a given universal set U. In such cases, is called the absolute complement or simply complement of A, and is denoted by A? or Ac.[15]
Examples:
Some basic properties of complements include the following:
An extension of the complement is the symmetric difference, defined for sets A, B as
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.
A new set can be constructed by associating every element of one set with every element of another set. The Cartesian product of two sets A and B, denoted by A × B,[15] is the set of all ordered pairs (a, b) such that a is a member of A and b is a member of B.
Examples:
Some basic properties of Cartesian products:
Let A and B be finite sets; then the cardinality of the Cartesian product is the product of the cardinalities:
Set theory is seen as the foundation from which virtually all of mathematics can be derived. For example, structures in abstract algebra, such as groups, fields and rings, are sets closed under one or more operations.
One of the main applications of naive set theory is in the construction of relations. A relation from a domain A to a codomain B is a subset of the Cartesian product A × B. For example, considering the set S = { rock, paper, scissors } of shapes in the game of the same name, the relation "beats" from S to S is the set B = { (scissors,paper), (paper,rock), (rock,scissors) }; thus x beats y in the game if the pair (x,y) is a member of B. Another example is the set F of all pairs (x, x2), where x is real. This relation is a subset of R' × R, because the set of all squares is subset of the set of all real numbers. Since for every x in R, one and only one pair (x,...) is found in F, it is called a function. In functional notation, this relation can be written as F(x) = x2.
Although initially naive set theory, which defines a set merely as any well-defined collection, was well accepted, it soon ran into several obstacles. It was found that this definition spawned several paradoxes, most notably:
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.
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 more general form of the principle can be used to find the cardinality of any finite union of sets:
Augustus De Morgan stated two laws about sets.
If A and B are any two sets then,
The complement of A union B equals the complement of A intersected with the complement of B.
The complement of A intersected with B is equal to the complement of A union to the complement of B.