Surjective function
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mathematics Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented in modern mathematics ...
, a surjective function (also known as surjection, or onto function) is a function that every element can be mapped from element so that . In other words, every element of the function's codomain is the
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of one element of its domain. It is not required that be unique; the function may map one or more elements of to the same element of . The term ''surjective'' and the related terms '' injective'' and '' bijective'' were introduced by Nicolas Bourbaki, a group of mainly French 20th-century
mathematician A mathematician is someone who uses an extensive knowledge of mathematics in their work, typically to solve mathematical problems. Mathematicians are concerned with numbers, data, quantity, mathematical structure, structure, space, Mathematica ...
s who, under this pseudonym, wrote a series of books presenting an exposition of modern advanced mathematics, beginning in 1935. The French word '' sur'' means ''over'' or ''above'', and relates to the fact that the
image An image is a visual representation of something. It can be two-dimensional, three-dimensional, or somehow otherwise feed into the visual system to convey information. An image can be an artifact, such as a photograph or other two-dimensiona ...
of the domain of a surjective function completely covers the function's codomain. Any function induces a surjection by restricting its codomain to the image of its domain. Every surjective function has a right inverse assuming the
axiom of choice In mathematics, the axiom of choice, or AC, is an axiom of set theory equivalent to the statement that ''a Cartesian product#Infinite Cartesian products, Cartesian product of a collection of non-empty sets is non-empty''. Informally put, the a ...
, and every function with a right inverse is necessarily a surjection. The composition of surjective functions is always surjective. Any function can be decomposed into a surjection and an injection.


Definition

A surjective function is a function whose
image An image is a visual representation of something. It can be two-dimensional, three-dimensional, or somehow otherwise feed into the visual system to convey information. An image can be an artifact, such as a photograph or other two-dimensiona ...
is equal to its codomain. Equivalently, a function f with domain X and codomain Y is surjective if for every y in Y there exists at least one x in X with f(x)=y. Surjections are sometimes denoted by a two-headed rightwards arrow (), as in f\colon X\twoheadrightarrow Y. Symbolically, :If f\colon X \rightarrow Y, then f is said to be surjective if and only if :\forall y \in Y, \, \exists x \in X, \;\; f(x)=y.


Examples

* For any set ''X'', the
identity function Graph of the identity function on the real numbers In mathematics Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantitie ...
id''X'' on ''X'' is surjective. * The function defined by ''f''(''n'') = ''n'' mod 2 (that is, even
integer An integer is the number zero (), a positive natural number (, , , etc.) or a negative integer with a minus sign (−1, −2, −3, etc.). The negative numbers are the additive inverses of the corresponding positive numbers. In the language of ...
s are mapped to 0 and odd integers to 1) is surjective. * The function defined by ''f''(''x'') = 2''x'' + 1 is surjective (and even bijective), because for every
real number In mathematics, a real number is a number that can be used to measurement, measure a ''continuous'' one-dimensional quantity such as a distance, time, duration or temperature. Here, ''continuous'' means that values can have arbitrarily small var ...
''y'', we have an ''x'' such that ''f''(''x'') = ''y'': such an appropriate ''x'' is (''y'' − 1)/2. * The function defined by ''f''(''x'') = ''x''3 − 3''x'' is surjective, because the pre-image of any
real number In mathematics, a real number is a number that can be used to measurement, measure a ''continuous'' one-dimensional quantity such as a distance, time, duration or temperature. Here, ''continuous'' means that values can have arbitrarily small var ...
''y'' is the solution set of the cubic polynomial equation ''x''3 − 3''x'' − ''y'' = 0, and every cubic polynomial with real coefficients has at least one real root. However, this function is not injective (and hence not bijective), since, for example, the pre-image of ''y'' = 2 is . (In fact, the pre-image of this function for every ''y'', −2 ≤ ''y'' ≤ 2 has more than one element.) * The function defined by ''g''(''x'') = ''x''2 is ''not'' surjective, since there is no real number ''x'' such that ''x''2 = −1. However, the function defined by (with the restricted codomain) ''is'' surjective, since for every ''y'' in the nonnegative real codomain ''Y'', there is at least one ''x'' in the real domain ''X'' such that ''x''2 = ''y''. * The
natural logarithm The natural logarithm of a number is its logarithm to the base (exponentiation), base of the mathematical constant , which is an Irrational number, irrational and Transcendental number, transcendental number approximately equal to . The natur ...
function is a surjective and even bijective (mapping from the set of positive real numbers to the set of all real numbers). Its inverse, the
exponential function The exponential function is a mathematical Function (mathematics), function denoted by f(x)=\exp(x) or e^x (where the argument is written as an exponentiation, exponent). Unless otherwise specified, the term generally refers to the positiv ...
, if defined with the set of real numbers as the domain, is not surjective (as its range is the set of positive real numbers). * The matrix exponential is not surjective when seen as a map from the space of all ''n''×''n'' matrices to itself. It is, however, usually defined as a map from the space of all ''n''×''n'' matrices to the
general linear group In mathematics, the general linear group of degree ''n'' is the set of invertible matrix, invertible matrices, together with the operation of ordinary matrix multiplication. This forms a group (mathematics), group, because the product of two in ...
of degree ''n'' (that is, the group of all ''n''×''n'' invertible matrices). Under this definition, the matrix exponential is surjective for complex matrices, although still not surjective for real matrices. * The projection from a
cartesian product In mathematics, specifically set theory, the Cartesian product of two Set (mathematics), sets ''A'' and ''B'', denoted ''A''×''B'', is the set of all ordered pairs where ''a'' is in ''A'' and ''b'' is in ''B''. In terms of set-builder notatio ...
to one of its factors is surjective, unless the other factor is empty. * In a 3D video game, vectors are projected onto a 2D flat screen by means of a surjective function.


Properties

A function is bijective if and only if it is both surjective and injective. If (as is often done) a function is identified with its graph, then surjectivity is not a property of the function itself, but rather a property of the mapping. This is, the function together with its codomain. Unlike injectivity, surjectivity cannot be read off of the graph of the function alone.


Surjections as right invertible functions

The function is said to be a right inverse of the function if ''f''(''g''(''y'')) = ''y'' for every ''y'' in ''Y'' (''g'' can be undone by ''f''). In other words, ''g'' is a right inverse of ''f'' if the composition of ''g'' and ''f'' in that order is the
identity function Graph of the identity function on the real numbers In mathematics Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantitie ...
on the domain ''Y'' of ''g''. The function ''g'' need not be a complete inverse of ''f'' because the composition in the other order, , may not be the identity function on the domain ''X'' of ''f''. In other words, ''f'' can undo or "''reverse''" ''g'', but cannot necessarily be reversed by it. Every function with a right inverse is necessarily a surjection. The proposition that every surjective function has a right inverse is equivalent to the
axiom of choice In mathematics, the axiom of choice, or AC, is an axiom of set theory equivalent to the statement that ''a Cartesian product#Infinite Cartesian products, Cartesian product of a collection of non-empty sets is non-empty''. Informally put, the a ...
. If is surjective and ''B'' is a
subset In mathematics, Set (mathematics), set ''A'' is a subset of a set ''B'' if all Element (mathematics), elements of ''A'' are also elements of ''B''; ''B'' is then a superset of ''A''. It is possible for ''A'' and ''B'' to be equal; if they are ...
of ''Y'', then ''f''(''f'' −1(''B'')) = ''B''. Thus, ''B'' can be recovered from its preimage . For example, in the first illustration above, there is some function ''g'' such that ''g''(''C'') = 4. There is also some function ''f'' such that ''f''(4) = ''C''. It doesn't matter that ''g''(''C'') can also equal 3; it only matters that ''f'' "reverses" ''g''.


Surjections as epimorphisms

A function is surjective if and only if it is right-cancellative: given any functions , whenever ''g'' o ''f'' = ''h'' o ''f'', then ''g'' = ''h''. This property is formulated in terms of functions and their composition and can be generalized to the more general notion of the morphisms of a
category Category, plural categories, may refer to: Philosophy and general uses *Categorization, categories in cognitive science, information science and generally *Category of being *Categories (Aristotle), ''Categories'' (Aristotle) *Category (Kant) ...
and their composition. Right-cancellative morphisms are called epimorphisms. Specifically, surjective functions are precisely the epimorphisms in the category of sets. The prefix ''epi'' is derived from the Greek preposition ''ἐπί'' meaning ''over'', ''above'', ''on''. Any morphism with a right inverse is an epimorphism, but the converse is not true in general. A right inverse ''g'' of a morphism ''f'' is called a
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of ''f''. A morphism with a right inverse is called a split epimorphism.


Surjections as binary relations

Any function with domain ''X'' and codomain ''Y'' can be seen as a left-total and right-unique binary relation between ''X'' and ''Y'' by identifying it with its function graph. A surjective function with domain ''X'' and codomain ''Y'' is then a binary relation between ''X'' and ''Y'' that is right-unique and both left-total and right-total.


Cardinality of the domain of a surjection

The cardinality of the domain of a surjective function is greater than or equal to the cardinality of its codomain: If is a surjective function, then ''X'' has at least as many elements as ''Y'', in the sense of
cardinal number In mathematics Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented ...
s. (The proof appeals to the
axiom of choice In mathematics, the axiom of choice, or AC, is an axiom of set theory equivalent to the statement that ''a Cartesian product#Infinite Cartesian products, Cartesian product of a collection of non-empty sets is non-empty''. Informally put, the a ...
to show that a function satisfying ''f''(''g''(''y'')) = ''y'' for all ''y'' in ''Y'' exists. ''g'' is easily seen to be injective, thus the formal definition of , ''Y'', ≤ , ''X'', is satisfied.) Specifically, if both ''X'' and ''Y'' are finite with the same number of elements, then is surjective if and only if ''f'' is injective. Given two sets ''X'' and ''Y'', the notation is used to say that either ''X'' is empty or that there is a surjection from ''Y'' onto ''X''. Using the axiom of choice one can show that and together imply that , ''Y'', = , ''X'', , a variant of the Schröder–Bernstein theorem.


Composition and decomposition

The composition of surjective functions is always surjective: If ''f'' and ''g'' are both surjective, and the codomain of ''g'' is equal to the domain of ''f'', then is surjective. Conversely, if is surjective, then ''f'' is surjective (but ''g'', the function applied first, need not be). These properties generalize from surjections in the category of sets to any epimorphisms in any
category Category, plural categories, may refer to: Philosophy and general uses *Categorization, categories in cognitive science, information science and generally *Category of being *Categories (Aristotle), ''Categories'' (Aristotle) *Category (Kant) ...
. Any function can be decomposed into a surjection and an injection: For any function there exist a surjection and an injection such that ''h'' = ''g'' o ''f''. To see this, define ''Y'' to be the set of preimages where ''z'' is in . These preimages are disjoint and partition ''X''. Then ''f'' carries each ''x'' to the element of ''Y'' which contains it, and ''g'' carries each element of ''Y'' to the point in ''Z'' to which ''h'' sends its points. Then ''f'' is surjective since it is a projection map, and ''g'' is injective by definition.


Induced surjection and induced bijection

Any function induces a surjection by restricting its codomain to its range. Any surjective function induces a bijection defined on a quotient of its domain by collapsing all arguments mapping to a given fixed image. More precisely, every surjection can be factored as a projection followed by a bijection as follows. Let ''A''/~ be the equivalence classes of ''A'' under the following equivalence relation: ''x'' ~ ''y'' if and only if ''f''(''x'') = ''f''(''y''). Equivalently, ''A''/~ is the set of all preimages under ''f''. Let ''P''(~) : ''A'' → ''A''/~ be the projection map which sends each ''x'' in ''A'' to its equivalence class 'x''sub>~, and let ''f''''P'' : ''A''/~ → ''B'' be the well-defined function given by ''f''''P''( 'x''sub>~) = ''f''(''x''). Then ''f'' = ''f''''P'' o ''P''(~).


Space of surjections

Given fixed and , one can form the set of surjections . The cardinality of this set is one of the twelve aspects of Rota's
Twelvefold way In combinatorics, the twelvefold way is a systematic classification of 12 related enumerative problems concerning two finite sets, which include the classical problems of counting permutations, combinations, multisets, and partitions either ...
, and is given by , B, !\begin, A, \\, B, \end, where \begin, A, \\, B, \end denotes a Stirling number of the second kind.


Gallery

File:Surjective composition.svg, Surjective composition: the first function need not be surjective. File:Non-surjective function2.svg, alt=, Non-surjective functions in the Cartesian plane. Although some parts of the function are surjective, where elements ''y'' in ''Y'' do have a value ''x'' in ''X'' such that ''y'' = ''f''(''x''), some parts are not. Left: There is ''y''0 in ''Y'', but there is no ''x''0 in ''X'' such that ''y''0 = ''f''(''x''0). Right: There are ''y''1, ''y''2 and ''y''3 in ''Y'', but there are no ''x''1, ''x''2, and ''x''3 in ''X'' such that ''y''1 = ''f''(''x''1), ''y''2 = ''f''(''x''2), and ''y''3 = ''f''(''x''3). File:Surjective function.svg, alt=, Interpretation for surjective functions in the Cartesian plane, defined by the mapping ''f'' : ''X'' → ''Y'', where ''y'' = ''f''(''x''), ''X'' = domain of function, ''Y'' = range of function. Every element in the range is mapped onto from an element in the domain, by the rule ''f''. There may be a number of domain elements which map to the same range element. That is, every ''y'' in ''Y'' is mapped from an element ''x'' in ''X'', more than one ''x'' can map to the same ''y''. Left: Only one domain is shown which makes ''f'' surjective. Right: two possible domains ''X''1 and ''X''2 are shown.


See also

* Bijection, injection and surjection * Cover (algebra) * Covering map * Enumeration * Fiber bundle * Index set * Section (category theory)


References


Further reading

* {{Mathematical logic Functions and mappings Basic concepts in set theory Mathematical relations Types of functions