Surjective function

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A surjective function from domain X to codomain Y. The function is surjective because every point in the codomain is the value of f(x) for at least one point x in the domain.

In mathematics, a function f from a set X to a set Y is surjective (or onto), or a surjection, if for every y in Y, there is an x in X with f(x) = y; that is, each element of Y can be obtained by applying f to some element of X. (There might be multiple elements of X that are turned into the same element of Y by applying f.)

The term surjective and the related terms injective and bijective were introduced by Nicolas Bourbaki,^[1] a group of mainly French 20th-century mathematicians who wrote a series of books presenting an exposition of modern advanced mathematics, beginning in 1935. The French prefix sur means over or above and relates to the fact that the image of the domain of a surjective function completely covers the function's codomain.

Definition

A surjective function is a function whose image 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 : X ↠ Y.

Examples

A non-surjective function from domain X to codomain Y. The smaller oval inside Y is the image (also called range) of f. This function is not surjective, because the image does not fill the whole codomain. In other words, Y is colored in a two-step process: First, for every x in X, the point f(x) is colored green; Second, all the rest of the points in Y, that are not green, are colored blue. The function f is surjective only if there are no blue points.

For any set X, the identity function id_X on X is surjective.

The function f : Z → {0,1} defined by f(n) = n mod 2 and mapping even integers to 0 and odd integers to 1 is surjective.

The function f : R → R defined by f(x) = 2x + 1 is surjective (and even bijective), because for every real number y we have an x such that f(x) = y: an appropriate x is (y − 1)/2.

The function g : R → R defined by g(x) = x² is not surjective, because there is no real number x such that x² = −1. However, the function g : R → R⁺ defined by g(x) = x² (with restricted codomain) is surjective because for every y in the positive real codomain Y there is at least one x in the real domain X such that |x² = y.

The natural logarithm function ln : (0,+∞) → R 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, is not surjective as its range is the set of positive real numbers and its domain is usually defined to be the set of all 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 of degree n, i.e. 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 A × B to one of its factors is surjective.

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 relationship between the function and its codomain. Unlike injectivity, surjectivity cannot be read off of the graph of the function alone.

Surjections as right invertible functions

The function g : Y → X is said to be a right inverse of the function f : X → Y 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 f o g of g and f in that order is the identity function on the domain Y of g. The function g need not be a complete inverse of f because the composition in the other order, g o f, 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.

If f : X → Y is surjective and B is a subset of Y, then f(f ⁻¹(B)) = B. Thus, B can be recovered from its preimage f ⁻¹(B).

For example, in the first illustration, 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.

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  Another surjective function. (This one happens to be a bijection)

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  A non-surjective function. (This one happens to be an injection)

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  Surjective composition: the first function need not be surjective.

Surjections as epimorphisms

A function f : X → Y is surjective if and only if it is right-cancellative:^[2] given any functions g,h : Y → Z, 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 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 section 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 f : X → Y is a surjective function, then X has at least as many elements as Y, in the sense of cardinal numbers. (The proof appeals to the axiom of choice to show that a function g : Y → X 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 f : X → Y is surjective if and only if f is injective.

Composition and decomposition

The composite 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 f o g is surjective. Conversely, if f o g 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.

Any function can be decomposed into a surjection and an injection: For any function h : X → Z there exist a surjection f : X → Y and an injection g : Y → Z such that h = g o f. To see this, define Y to be the sets h ⁻¹(z) where z is in Z. These sets 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 f : A → B 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]_~, and let f_P : A/~ → B be the well-defined function given by f_P([x]_~) = f(x). Then f = f_P o P(~).

See also

• Cover (algebra)
• Covering map
• Enumeration
• Fiber bundle
• Index set
• Section (category theory)

Notes

1. ^ Earliest Uses of Some of the Words of Mathematics: entry on Injection, Surjection and Bijection has the history of surjection and related terms.
2. ^ Goldblatt, Robert (2006) [1984]. Topoi, the Categorial Analysis of Logic (Revised ed.). Dover Publications. ISBN 978-0486450261. http://historical.library.cornell.edu/cgi-bin/cul.math/docviewer?did=Gold010&id=3. Retrieved 2009-11-25. 

References

• Bourbaki, Nicolas (2004) [1968]. Theory of Sets. Springer. ISBN 978-3540225256.