, a set is a collection of distinct elements
The elements that make up a set can be any kind of things: people, letters of the alphabet, numbers, points in space, lines, other geometrical shapes, variables, or even other sets. Two sets are equal if and only if
they have precisely the same elements.
Sets are ubiquitous in modern mathematics. Indeed, set theory
, more specifically Zermelo–Fraenkel set theory
, has been the standard way to provide rigorous foundations
for all branches of mathematics since the first half of the 20th century.
The concept of a set emerged in mathematics at the end of the 19th century.
The German word for set, ''Menge'', was coined by Bernard Bolzano
in his work ''Paradoxes of the Infinite
, one of the founders of set theory, gave the following definition at the beginning of his ''Beiträge zur Begründung der transfiniten Mengenlehre'':
Naïve set theory
The foremost property of a set is that it can have elements, also called ''members''. Two sets are equal
when they have the same elements. More precisely, sets ''A'' and ''B'' are equal if every element of ''A'' is a member of ''B'', and every element of ''B'' is an element of ''A''; this property is called the ''extensionality
The simple concept of a set has proved enormously useful in mathematics, but paradoxes
arise if no restrictions are placed on how sets can be constructed:
* Russell's paradox
shows that the "set of all sets that ''do not contain themselves''", i.e., , cannot exist.
* Cantor's paradox
shows that "the set of all sets" cannot exist.
Naïve set theory
defines a set as any ''well-defined
'' collection of distinct elements, but problems arise from the vagueness of the term ''well-defined''.
Axiomatic set theory
In subsequent efforts to resolve these paradoxes since the time of the original formulation of naïve set theory, the properties of sets have been defined by axioms
. Axiomatic set theory
takes the concept of a set as a primitive notion
The purpose of the axioms is to provide a basic framework from which to deduce the truth or falsity of particular mathematical propositions
(statements) about sets, using first-order logic
. According to Gödel's incompleteness theorems
however, it is not possible to use first-order logic to prove any such particular axiomatic set theory is free from paradox.
How sets are defined and set notation
Mathematical texts commonly denote sets by capital letters
, such as , , .
A set may also be called a ''collection'' or ''family'', especially when its elements are themselves sets.
One way to define a set is to use a rule to determine what the elements are:
:Let be the set whose members are the first four positive integers
:Let be the set of colors of the French flag
Such a definition is also called a ''semantic description''.
''Roster'' or ''enumeration notation'' defines a set by listing its elements between curly brackets
, separated by commas:
: = .
In a set, all that matters is whether each element is in it or not, so the ordering of the elements in roster notation is irrelevant (in contrast, in a sequence
, a tuple
, or a permutation of a set
, the ordering of the terms matters). For example, and represent the same set.
For sets with many elements, especially those following an implicit pattern, the list of members can be abbreviated using an ellipsis
For instance, the set of the first thousand positive integers may be specified in roster notation as
Infinite sets in roster notation
An infinite set
is a set with an endless list of elements. To describe an infinite set in roster notation, an ellipsis is placed at the end of the list, or at both ends, to indicate that the list continues forever. For example, the set of nonnegative integers
and the set of all integer
Set-builder notation specifies a set as a selection from a larger set, determined by a condition on the elements.
For example, a set can be defined as follows:
In this notation, the vertical bar
"|" means "such that", and the description can be interpreted as " is the set of all numbers such that is an integer in the range from 0 to 19 inclusive". Some authors use a colon
":" instead of the vertical bar.
Classifying methods of definition
uses specific terms to classify types of definitions:
*An ''intensional definition
'' uses a ''rule'' to determine membership. Semantic definitions and definitions using set-builder notation are examples.
*An ''extensional definition
'' describes a set by ''listing all its elements''.
Such definitions are also called ''enumerative
*An ''ostensive definition
'' is one that describes a set by giving ''examples'' of elements; a roster involving an ellipsis would be an example.
If ''B'' is a set and ''x'' is an element of ''B'', this is written in shorthand as ''x'' ∈ ''B'', which can also be read as ''"x belongs to B"'', or ''"x is in B"''. The statement ''"y is not an element of B"'' is written as ''y'' ∉ ''B'', which can also be read as or ''"y is not in B"''.
For example, with respect to the sets ''A'' = , ''B'' = , and ''F'' = ,
:4 ∈ ''A'' and 12 ∈ ''F''; and
:20 ∉ ''F'' and green ∉ ''B''.
The empty set
The ''empty set'' (or ''null set'') is the unique set that has no members. It is denoted ∅ or
A ''singleton set'' is a set with exactly one element; such a set may also be called a ''unit set''.
Any such set can be written as , where ''x'' is the element.
The set and the element ''x'' mean different things; Halmos draws the analogy that a box containing a hat is not the same as the hat.
If every element of set ''A'' is also in ''B'', then ''A'' is described as being a ''subset of B'', or ''contained in B'', written ''A'' ⊆ ''B''.
''B'' ⊇ ''A'' means ''B contains A'', ''B includes A'', or ''B is a superset of A''; ''B'' ⊇ ''A'' is equivalent to ''A'' ⊆ ''B''.
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''.
If ''A'' is a subset of ''B'', but ''A'' is not equal to ''B'', then ''A'' is called a ''proper subset'' of ''B''. This can be written ''A'' ⊊ ''B''. Likewise, ''B'' ⊋ ''A'' means ''B is a proper superset of A'', i.e. ''B'' contains ''A'', and is not equal to ''A''.
A third pair of operators ⊂ and ⊃ are used differently by different authors: some authors use ''A'' ⊂ ''B'' and ''B'' ⊃ ''A'' to mean ''A'' is any subset of ''B'' (and not necessarily a proper subset),
while others reserve ''A'' ⊂ ''B'' and ''B'' ⊃ ''A'' for cases where ''A'' is a proper subset of ''B''.
* The set of all humans is a proper subset of the set of all mammals.
* ⊂ .
* ⊆ .
The empty set is a subset of every set, and every set is a subset of itself:
* ∅ ⊆ ''A''.
* ''A'' ⊆ ''A''.
Euler and Venn diagrams
An Euler diagram
is a graphical representation of a collection of sets; each set is depicted as a planar region enclosed by a loop, with its elements inside. If is a subset of , then the region representing is completely inside the region representing . If two sets have no elements in common, the regions do not overlap.
A Venn diagram
, in contrast, is a graphical representation of sets in which the loops divide the plane into zones such that for each way of selecting some of the sets (possibly all or none), there is a zone for the elements that belong to all the selected sets and none of the others. For example, if the sets are , , and , there should be a zone for the elements that are inside and and outside (even if such elements do not exist).
Special sets of numbers in mathematics
There are sets of such mathematical importance, to which mathematicians refer so frequently, that they have acquired special names and notational conventions to identify them.
Many of these important sets are represented in mathematical texts using bold (e.g. ) or blackboard bold
, the set of all natural number
s: (some authors exclude );
, the set of all integer
s (whether positive, negative or zero): ;
, the set of all rational number
s (that is, the set of all proper
and improper fraction
s): . For example, and ;
, the set of all real numbers
, including all rational numbers and all irrational
numbers (which include algebraic number
s such as that cannot be rewritten as fractions, as well as transcendental numbers
, the set of all complex number
s: , for example, .
Each of the above sets of numbers has an infinite number of elements. Each is a subset of the sets listed below it.
Sets of positive or negative numbers are sometimes denoted by superscript plus and minus signs, respectively. For example,
represents the set of positive rational numbers.
'' (or ''mapping
'') from a set to a set is a rule that assigns to each "input" element of an "output" that is an element of ; more formally, a function is a special kind of relation
, one that relates each element of to ''exactly one'' element of . A function is called
(or one-to-one) if it maps any two different elements of to ''different'' elements of ,
(or onto) if for every element of , there is at least one element of that maps to it, and
(or a one-to-one correspondence) if the function is both injective and surjective — in this case, each element of is paired with a unique element of , and each element of is paired with a unique element of , so that there are no unpaired elements.
An injective function is called an ''injection'', a surjective function is called a ''surjection'', and a bijective function is called a ''bijection'' or ''one-to-one correspondence''.
The cardinality of a set ''S'', denoted , is the number of members of ''S''.
For example, if ''B'' = , then . Repeated members in roster notation are not counted,
so , too.
More formally, two sets share the same cardinality if there exists a one-to-one correspondence between them.
The cardinality of the empty set is zero.
Infinite sets and infinite cardinality
The list of elements of some sets is endless, or ''infinite
''. For example, the set
of natural numbers
In fact, all the special sets of numbers mentioned in the section above, are infinite. Infinite sets have ''infinite cardinality''.
Some infinite cardinalities are greater than others. Sets with the same cardinality as
are called ''countable sets
''. Arguably one of the most significant results from set theory is that the set of real numbers
has greater cardinality than the set of natural numbers.
Sets with cardinality greater than the set of natural numbers are called uncountable sets
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
The Continuum Hypothesis
The Continuum Hypothesis, formulated by Georg Cantor in 1878, is the statement that there is no set with cardinality strictly between the cardinality of the natural numbers
and the cardinality of a straight line.
In 1963, Paul Cohen
proved that the Continuum Hypothesis is independent
of the axiom system ZFC consisting of Zermelo–Fraenkel set theory
with the axiom of choice
(ZFC is the most widely-studied version of axiomatic set theory.)
The power set of a set ''S'' is the set of all subsets of ''S''.
The empty set
and ''S'' itself are elements of the power set of ''S'' because these are both subsets of ''S''. For example, the power set of is . The power set of a set ''S'' is commonly written as ''P''(''S'') or 2''P''
The power set of a finite set with ''n'' elements has 2''n''
elements. For example, the set contains three elements, and the power set shown above contains 23
= 8 elements.
The power set of an infinite (either countable
) set is always uncountable. Moreover, within the most widely-used frameworks of set theory, 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'').)
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''.
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'',
is the set of all things that are members of either ''A'' or ''B''.
* ∪ =
Some basic properties of unions:
* if and only if
A new set can also be constructed by determining which members two sets have "in common". The ''intersection'' of ''A'' and ''B'', denoted by
is the set of all things that are members of both ''A'' and ''B''. If then ''A'' and ''B'' are said to be ''disjoint''.
Some basic properties of intersections:
* if and only if
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 ),
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 ; 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
* 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 include the following:
* for .
* if then
An extension of the complement is the symmetric difference
, defined for sets ''A'', ''B'' as
For example, the symmetric difference of and is the set . 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,''
is the set of all ordered pairs
(''a'', ''b'') such that ''a'' is a member of ''A'' and ''b'' is a member of ''B''.
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:
* | ''A'' × ''B'' | = | ''B'' × ''A'' | = | ''A'' | × | ''B'' |.
Sets are ubiquitous in modern mathematics. For example, structures
in abstract algebra
, such as groups
, 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'' = of shapes in the game
of the same name, the relation "beats" from ''S'' to ''S'' is the set ''B'' = ; 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'', ''x''2
), 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'') = ''x''2
Principle of inclusion and exclusion
thumb|The inclusion-exclusion principle is 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 more general form of the principle can be used to find the cardinality of any finite union of sets:
De Morgan's laws
Augustus De Morgan
stated two laws
If A and B are any two sets then,
* (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.
* Algebra of sets
* Alternative set theory
* Axiomatic set theory
* Category of sets
* Class (set theory)
* Dense set
* Family of sets
* Fuzzy set
* Internal set
* Mathematical object
* Naive set theory
* Principia Mathematica
* Rough set
* Russell's paradox
* Sequence (mathematics)
* Set notation
* Venn diagram
Cantor's "Beiträge zur Begründung der transfiniten Mengenlehre" (in German)
Category:Concepts in logic