# Function composition

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For function composition in computer science, see function composition (computer science).

In mathematics, function composition is the pointwise application of one function to the result of another to produce a third function. For instance, the functions f : XY and g : YZ can be composed to yield a function which maps x in X to g(f(x)) in Z. Intuitively, if z is a function g of y, and y is a function f of x, then z is a function of x. The resulting composite function is notated g ∘ f : XZ, 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", or "g round f", or "g composed with f", "g after f", "g following f", or "g of f".

The composition of functions is always associative. 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 : (−∞,+9] defined by f(x) = 9 - x2 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. In general, composition of functions will not be commutative. 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.

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.

The composition of one-to-one functions is always one-to-one. Similarly, the composition of two onto functions is always onto.

## Examples

g ∘ f, the composition of f and g. For example, (g ∘ f )(c) = #.
Compositions of two real functions, absolute value and a cubic function, in different orders show a non-commutativity of the composition.
• 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 gf = {(1,4), (2,5), (3,1), (4,2)}.
• Composition of functions on an infinite set: If f: RR is given by f(x) = 2x + 4 and g: RR is given by g(x) = x3, then:
(fg)(x) = f(g(x)) = f(x3) = 2x3 + 4, and
(gf)(x) = g(f(x)) = g(2x + 4) = (2x + 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 (ch)(t) describes the oxygen concentration around the plane at time t.

## Functional powers

Main article: Iterated function

If Y X, then f: XY may compose with itself; this is sometimes denoted as f 2. That is:

(ff)(x) = f(f(x)) = f 2(x)
(fff)(x) = f(f(f(x))) = f 3(x)

More generally, for any natural number n≥2, the nth functional power can be defined inductively by f n = ff n-1 = f n-1f. 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, idX.
• If even Y=X and f: XX admits an inverse function f -1, negative functional powers f -n are defined for n>0 as the opposite 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: sin2(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 gg = f has a unique solution g, that function can be defined as the functional square root of f, then written as g = f ½. More generally, when gn = f has a unique solution for some natural number n>0, then f m/n can be defined as gm. 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.

## Composition monoids

Main article: Transformation monoid

Suppose one has two (or more) functions f: XX, g: XX having the same domain and codomain. Then one can form chains of these functions composed together, such as ffgf. Such chains have the algebraic structure of a monoid, called a transformation monoid or composition monoid. In general, composition monoids can have remarkably complicated structure. One particular notable example is the de Rham curve. The set of all functions f: XX is called the full transformation semigroup on X.

If the functions are bijective, 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.

The set of all bijective functions f: XX forms a group with respect to the composition operator. This is the symmetric group, also sometimes called the composition group.

## Alternative notations

The similarity that transforms triangle EFA into triangle ATB is the composition of a homothety H  and a rotation R, of which the common centre is S.  For example, the image of  under the rotation R is U,  which may be written  R (A) = U.  And  H(U) = B  means that the mapping H transforms U  into B.  Thus  H(R (A)) = (H ∘ R )(A) = B.
• Many mathematicians omit the composition symbol, writing gf for gf.
• In the mid-20th century, some mathematicians decided that writing "gf" 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))". 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 matrix multiplication is non-commutative. 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 do f then do 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, thereby disambiguating the order of composition. To distinguish the left composition operator from a text semicolon, in the Z notation the U+2A1F fat semicolon character is used for left relation composition. 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 operator

Main article: Composition operator

Given a function g, the composition operator Cg is defined as that operator which maps functions to functions as

$C_g f = f \circ g.$

Composition operators are studied in the field of operator theory.

## Generalizations

The structures given by composition are axiomatized and generalized in category theory with the concept of morphism as the category-theoretical replacement of functions.

Composition can be generalized to arbitrary binary relations. If RX × Y and SY × Z are two binary relations, then their composition SR is the relation defined as {(x,z) ∈ X×Z : yY. (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.

Composition is possible for multivariate functions. The function resulting when some argument xi of the function f is replaced by the function g is called a composition of f and g, and is denoted f |xi = g

$f|_{x_i = g} = f (x_1, \ldots, x_{i-1}, g(x_1, x_2, \ldots, x_n), x_{i+1}, \ldots, x_n).$

When g is a simple constant b, composition degenerates into a (partial) valuation, whose result is also known as restriction or co-factor[1]

$f|_{x_i = b} = f (x_1, \ldots, x_{i-1}, b, x_{i+1}, \ldots, x_n).$