# Function composition

In mathematics, **function composition** is an operation that takes two functions *f* and *g* and produces a function *h* such that *h*(*x*) = *g*(*f*(*x*)). In this operation, the function *g* is applied to the result of applying the function *f* to *x*. That is, the functions *f* : *X* → *Y* and *g* : *Y* → *Z* are **composed** to yield a function that maps *x* in *X* to *g*(*f*(*x*)) in *Z*.

Intuitively, if *z* is a function of *y*, and *y* is a function of *x*, then *z* is a function of *x*. The resulting *composite* function is denoted *g* ∘ *f* : *X* → *Z*, defined by (*g* ∘ *f* )(*x*) = *g*(*f*(*x*)) for all *x* in *X*.^{[note 1]}
The notation *g* ∘ *f* is read as "*g* circle *f* ", "*g* round *f* ", "*g* about *f* ", "*g* composed with *f* ", "*g* after *f* ", "*g* following *f* ", "*g* of *f*", or "*g* on *f* ". Intuitively, composing two functions is a chaining process in which the output of the inner function becomes the input of the outer function.

The composition of functions is a special case of the composition of relations, so all properties of the latter are true of composition of functions.^{[1]} The composition of functions has some additional properties.

## ExamplesEdit

- Composition of functions on a finite set: If
*f*= {(1, 3), (2, 1), (3, 4), (4, 6)}, and*g*= {(1, 5), (2, 3), (3, 4), (4, 1), (5, 3), (6, 2)}, then*g*∘*f*= {(1, 4), (2, 5), (3, 1), (4, 2)}. - Composition of functions on an infinite set: If
*f*: ℝ → ℝ (where ℝ is the set of all real numbers) is given by*f*(*x*) = 2*x*+ 4 and*g*: ℝ → ℝ is given by*g*(*x*) =*x*^{3}, then:

- (
*f*∘*g*)(*x*) =*f*(*g*(*x*)) =*f*(*x*^{3}) = 2*x*^{3}+ 4, and - (
*g*∘*f*)(*x*) =*g*(*f*(*x*)) =*g*(2*x*+ 4) = (2*x*+ 4)^{3}.

- If an airplane's elevation at time
*t*is given by the function*h*(*t*), and the oxygen concentration at elevation*x*is given by the function*c*(*x*), then (*c*∘*h*)(*t*) describes the oxygen concentration around the plane at time*t*.

## PropertiesEdit

The composition of functions is always associative—a property inherited from the composition of relations.^{[1]} That is, if *f*, *g*, and *h* are three functions with suitably chosen domains and codomains, then *f* ∘ (*g* ∘ *h*) = (*f* ∘ *g*) ∘ *h*, where the parentheses serve to indicate that composition is to be performed first for the parenthesized functions. Since there is no distinction between the choices of placement of parentheses, they may be left off without causing any ambiguity.

In a strict sense, the composition *g* ∘ *f* can be built only if *f*'s codomain equals *g*'s domain; in a wider sense it is sufficient that the former is a subset of the latter.^{[note 2]}
Moreover, it is often convenient to tacitly restrict *f*'s domain such that *f* produces only values in *g*'s domain; for example, the composition *g* ∘ *f* of the functions *f* : ℝ → (−∞,+9] defined by *f*(*x*) = 9 − *x*^{2} and *g* : [0,+∞) → ℝ defined by *g*(*x*) = √*x* can be defined on the interval [−3,+3].

The functions *g* and *f* are said to commute with each other if *g* ∘ *f* = *f* ∘ *g*. Commutativity is a special property, attained only by particular functions, and often in special circumstances. For example, |*x*| + 3 = |*x* + 3| only when *x* ≥ 0. The picture shows another example.

The composition of one-to-one functions is always one-to-one. Similarly, the composition of two onto functions is always onto. It follows that composition of two bijections is also a bijection. The inverse function of a composition (assumed invertible) has the property that (*f* ∘ *g*)^{−1} = ( *g*^{−1} ∘ *f* ^{−1}).^{[2]}

Derivatives of compositions involving differentiable functions can be found using the chain rule. Higher derivatives of such functions are given by Faà di Bruno's formula.

## Composition monoidsEdit

Suppose one has two (or more) functions *f*: *X* → *X*, *g*: *X* → *X* having the same domain and codomain; these are often called *transformations*. Then one can form chains of transformations composed together, such as *f* ∘ *f* ∘ *g* ∘ *f*. Such chains have the algebraic structure of a monoid, called a transformation monoid or (much more seldom) a *composition monoid*. In general, transformation monoids can have remarkably complicated structure. One particular notable example is the de Rham curve. The set of *all* functions *f*: *X* → *X* is called the full transformation semigroup^{[3]} or *symmetric semigroup*^{[4]} on *X*. (One can actually define two semigroups depending how one defines the semigroup operation as the left or right composition of functions.^{[5]})

If the transformations are bijective (and thus invertible), then the set of all possible combinations of these functions forms a transformation group; and one says that the group is generated by these functions. A fundamental result in group theory, Cayley's theorem, essentially says that any group is in fact just a subgroup of a permutation group (up to isomorphism).^{[6]}

The set of all bijective functions *f*: *X* → *X* (called permutations) forms a group with respect to the composition operator. This is the symmetric group, also sometimes called the *composition group*.

In the symmetric semigroup (of all transformations) one also finds a weaker, non-unique notion of inverse (called a pseudoinverse) because the symmetric semigroup is a regular semigroup.^{[7]}

## Functional powersEdit

If *Y* ⊆ *X*, then *f*: *X*→*Y* may compose with itself; this is sometimes denoted as *f*^{ 2}. That is:

- (
*f*∘*f*)(x) =*f*(*f*(*x*)) =*f*^{2}(*x*)

- (
*f*∘*f*∘*f*)(x) =*f*(*f*(*f*(*x*))) =*f*^{3}(*x*)

- (
*f*∘*f*∘*f*∘*f*)(x) =*f*(*f*(*f*(*f*(*x*)))) =*f*^{4}(*x*)

More generally, for any natural number *n* ≥ 2, the *n*th **functional power** can be defined inductively by *f* ^{n} = *f* ∘ *f* ^{n−1} = *f* ^{n−1} ∘ *f*. Repeated composition of such a function with itself is called **iterated function**.

- By convention,
*f*^{0}is defined as the identity map on*f*'s domain,*id*_{X}. - If even
*Y*=*X*and*f*:*X*→*X*admits an inverse function*f*^{−1}, negative functional powers*f*^{−n}are defined for*n*> 0 as the negated power of the inverse function:*f*^{−n}= (*f*^{−1})^{n}.

**Note:** If *f* takes its values in a ring (in particular for real or complex-valued *f* ), there is a risk of confusion, as *f* ^{n} could also stand for the *n*-fold product of *f*, e.g. *f* ^{2}(*x*) = *f*(*x*) · *f*(*x*). For trigonometric functions, usually the latter is meant, at least for positive exponents. For example, in trigonometry, this superscript notation represents standard exponentiation when used with trigonometric functions:
sin^{2}(*x*) = sin(*x*) · sin(*x*).
However, for negative exponents (especially −1), it nevertheless usually refers to the inverse function, e.g., tan^{−1} = arctan ≠ 1/tan.

In some cases, when, for a given function *f*, the equation *g* ∘ *g* = *f* has a unique solution *g*, that function can be defined as the functional square root of *f*, then written as *g* = *f* ^{1/2}.

More generally, when *g*^{n} = *f* has a unique solution for some natural number *n* > 0, then *f* ^{m/n} can be defined as *g*^{m}.

Under additional restrictions, this idea can be generalized so that the iteration count becomes a continuous parameter; in this case, such a system is called a flow, specified through solutions of Schröder's equation. Iterated functions and flows occur naturally in the study of fractals and dynamical systems.

To avoid ambiguity, some mathematicians choose to write *f* °^{n} for the *n*-th iterate of the function *f*.

## Alternative notationsEdit

Many mathematicians, particularly in group theory, omit the composition symbol, writing *gf* for *g* ∘ *f*.^{[8]}

In the mid-20th century, some mathematicians decided that writing "*g* ∘ *f* " to mean "first apply *f*, then apply *g*" was too confusing and decided to change notations. They write "*xf* " for "*f*(*x*)" and "(*xf*)*g*" for "*g*(*f*(*x*))".^{[9]} This can be more natural and seem simpler than writing functions on the left in some areas – in linear algebra, for instance, when *x* is a row vector and *f* and *g* denote matrices and the composition is by matrix multiplication. This alternative notation is called postfix notation. The order is important because function composition is not necessarily commutative (e.g. matrix multiplication). Successive transformations applying and composing to the right agrees with the left-to-right reading sequence.

Mathematicians who use postfix notation may write "*fg*", meaning first apply *f* and then apply *g*, in keeping with the order the symbols occur in postfix notation, thus making the notation "*fg*" ambiguous. Computer scientists may write "*f* ; *g*" for this,^{[10]} thereby disambiguating the order of composition. To distinguish the left composition operator from a text semicolon, in the Z notation the ⨾ character is used for left relation composition.^{[11]} Since all functions are binary relations, it is correct to use the [fat] semicolon for function composition as well (see the article on composition of relations for further details on this notation).

## Composition operatorEdit

Given a function *g*, the **composition operator** *C*_{g} is defined as that operator which maps functions to functions as

Composition operators are studied in the field of operator theory.

## In programming languagesEdit

Function composition appears in one form or another in numerous programming languages.

## Multivariate functionsEdit

Partial composition is possible for multivariate functions. The function resulting when some argument *x*_{i} of the function *f* is replaced by the function *g* is called a composition of *f* and *g* in some computer engineering contexts, and is denoted *f* |_{xi = g}

When *g* is a simple constant *b*, composition degenerates into a (partial) valuation, whose result is also known as restriction or *co-factor*.^{[12]}

In general, the composition of multivariate functions may involve several other functions as arguments, as in the definition of primitive recursive function. Given *f*, a *n*-ary function, and *n* *m*-ary functions *g*_{1}, ..., *g*_{n}, the composition of *f* with *g*_{1}, ..., *g*_{n}, is the *m*-ary function

- .

This is sometimes called the **generalized composite** of *f* with *g*_{1}, ..., *g*_{n}.^{[13]} The partial composition in only one argument mentioned previously can be instantiated from this more general scheme by setting all argument functions except one to be suitably chosen projection functions. Here *g*_{1}, ..., *g*_{n} can be seen as a single vector/tuple-valued function in this generalized scheme, in which case this is precisely the standard definition of function composition.^{[14]}

A set of finitary operations on some base set *X* is called a clone if it contains all projections and is closed under generalized composition. Note that a clone generally contains operations of various arities.^{[13]} The notion of commutation also finds an interesting generalization in the multivariate case; a function *f* of arity *n* is said to commute with a function *g* of arity *m* if *f* is a homomorphism preserving *g*, and vice versa i.e.:^{[15]}

- .

A unary operation always commutes with itself, but this is not necessarily the case for a binary (or higher arity) operation. A binary (or higher arity) operation that commutes with itself is called medial or entropic.^{[15]}

## GeneralizationsEdit

Composition can be generalized to arbitrary binary relations.
If *R* ⊆ *X* × *Y* and *S* ⊆ *Y* × *Z* are two binary relations, then their composition *S*∘*R* is the relation defined as {(*x*, *z*) ∈ *X* × *Z* : ∃*y* ∈ *Y*. (*x*, *y*) ∈ *R* ∧ (*y*, *z*) ∈ *S*}.
Considering a function as a special case of a binary relation (namely functional relations), function composition satisfies the definition for relation composition.

The composition is defined in the same way for partial functions and Cayley's theorem has its analogue called the Wagner-Preston theorem.^{[16]}

The category of sets with functions as morphisms is the prototypical category. The axioms of a category are in fact inspired from the properties (and also the definition) of function composition.^{[17]} The structures given by composition are axiomatized and generalized in category theory with the concept of morphism as the category-theoretical replacement of functions. The reversed order of composition in the formula (*f* ∘ *g*)^{−1} = (*g*^{−1} ∘ *f* ^{−1}) applies for composition of relations using converse relations, and thus in group theory. These structures form dagger categories.

## TypographyEdit

The composition symbol ∘ is encoded as U+2218 ∘ RING OPERATOR (HTML `∘`

); see the Degree symbol article for similar-appearing Unicode characters. In TeX, it is written `\circ`

.

## See alsoEdit

- Combinatory logic
- Function composition (computer science)
- Functional decomposition
- Iterated function
- Infinite compositions of analytic functions
- Flow (mathematics)
- Higher-order function
- Cobweb plot – a graphical technique for functional composition
- Lambda calculus
- Functional square root
- Composition ring, a formal axiomatization of the composition operation
- Function of random variable, distribution of a function of a random variable

## NotesEdit

**^**Some authors use*f*∘*g*:*X*→*Z*, defined by (*f*∘*g*)(*x*) =*g*(*f*(*x*)) instead. This is common when a postfix notation is used, especially if functions are represented by exponents, as, for instance, in the study of group actions. See Dixon, John D.; Mortimer, Brian (1996),*Permutation groups*, Springer, p. 5, ISBN 0-387-94599-7**^**The strict sense is used,*e.g.*, in category theory, where a subset relation is modelled explicitly by an inclusion function.

## ReferencesEdit

- ^
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^{a}^{b}Clifford Bergman (2011).*Universal Algebra: Fundamentals and Selected Topics*. CRC Press. pp. 79–80. ISBN 978-1-4398-5129-6. **^**George Tourlakis (2012).*Theory of Computation*. John Wiley & Sons. p. 100. ISBN 978-1-118-31533-0.- ^
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## External linksEdit

- Hazewinkel, Michiel, ed. (2001) [1994], "Composite function",
*Encyclopedia of Mathematics*, Springer Science+Business Media B.V. / Kluwer Academic Publishers, ISBN 978-1-55608-010-4 - "Composition of Functions" by Bruce Atwood, the Wolfram Demonstrations Project, 2007.