Function composition


In mathematics, function composition is an operation that takes two functions and and produces a function such that. In this operation, the function is applied to the result of applying the function to. That is, the functions and are composed to yield a function that maps in to in.
Intuitively, if is a function of, and is a function of, then is a function of. The resulting composite function is denoted, defined by for all in .
The notation is read as " circle ", " round ", " about ", " composed with ", " after ", " following ", " of ", or " on ". Intuitively, composing functions is a chaining process in which the output of function feeds the input of 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. The composition of functions has some additional properties.

Examples

The composition of functions is always associative—a property inherited from the composition of relations. That is, if,, and are three functions with suitably chosen domains and codomains, then, 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 can be built only if the codomain of equals the domain of ; in a wider sense it is sufficient that the former be a subset of the latter.
Moreover, it is often convenient to tacitly restrict the domain of such that produces only values in the domain of ; for example, the composition of the functions defined by and defined by can be defined on the interval.
and a cubic function, in different orders, show a non-commutativity of composition.
The functions and are said to commute with each other if. Commutativity is a special property, attained only by particular functions, and often in special circumstances. For example, only when. The picture shows another example.
The composition of one-to-one functions is always one-to-one. Similarly, the composition of onto functions is always onto. It follows that the composition of two bijections is also a bijection. The inverse function of a composition has the property that.
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 monoids

Suppose one has two functions having the same domain and codomain; these are often called transformations. Then one can form chains of transformations composed together, such as. Such chains have the algebraic structure of a monoid, called a transformation monoid or 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 is called the full transformation semigroup or symmetric semigroup on .
If the transformations 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. A fundamental result in group theory, Cayley's theorem, essentially says that any group is in fact just a subgroup of a permutation group.
The set of all bijective functions forms a group with respect to function composition. This is the symmetric group, also sometimes called the composition group.
In the symmetric semigroup one also finds a weaker, non-unique notion of inverse because the symmetric semigroup is a regular semigroup.

Functional powers

If, then may compose with itself; this is sometimes denoted as. That is:
More generally, for any natural number, the th functional power can be defined inductively by. Repeated composition of such a function with itself is called iterated function.
Note: If takes its values in a ring, there is a risk of confusion, as could also stand for the -fold product of , e.g.. 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:
However, for negative exponents, it nevertheless usually refers to the inverse function, e.g.,.
In some cases, when, for a given function, the equation has a unique solution, that function can be defined as the functional square root of, then written as.
More generally, when has a unique solution for some natural number, then can be defined as.
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 for the n-th iterate of the function.

Alternative notations

Many mathematicians, particularly in group theory, omit the composition symbol, writing for.
In the mid-20th century, some mathematicians decided that writing "" to mean "first apply, then apply " was too confusing and decided to change notations. They write "" for "" and "" for "". This can be more natural and seem simpler than writing functions on the left in some areas – in linear algebra, for instance, when is a row vector and and 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. Successive transformations applying and composing to the right agrees with the left-to-right reading sequence.
Mathematicians who use postfix notation may write "", meaning first apply and then apply, in keeping with the order the symbols occur in postfix notation, thus making the notation "" ambiguous. Computer scientists may write "" for this, 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. Since all functions are binary relations, it is correct to use the semicolon for function composition as well.

Composition operator

Given a function , the composition operator is defined as that operator which maps functions to functions as
Composition operators are studied in the field of operator theory.

In programming languages

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

Multivariate functions

Partial composition is possible for multivariate functions. The function resulting when some argument of the function is replaced by the function is called a composition of and in some computer engineering contexts, and is denoted
When is a simple constant, composition degenerates into a valuation, whose result is also known as restriction or co-factor.
In general, the composition of multivariate functions may involve several other functions as arguments, as in the definition of primitive recursive function. Given, a -ary function, and -ary functions, the composition of with, is the -ary function
This is sometimes called the generalized composite of f with. 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 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.
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. 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.:
A unary operation always commutes with itself, but this is not necessarily the case for a binary operation. A binary operation that commutes with itself is called medial or entropic.

Generalizations

can be generalized to arbitrary binary relations.
If and are two binary relations, then their composition is the relation defined as.
Considering a function as a special case of a binary relation, function composition satisfies the definition for relation composition. A small circle has been used for the infix notation of composition of relations, as well as functions. When used to represent composition of functions however, the text sequence is reversed to illustrate the different operation sequences accordingly.
The composition is defined in the same way for partial functions and Cayley's theorem has its analogue called the Wagner–Preston theorem.
The category of sets with functions as morphisms is the prototypical category. The axioms of a category are in fact inspired from the properties of function composition. 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 applies for composition of relations using converse relations, and thus in group theory. These structures form dagger categories.

Typography

The composition symbol is encoded as ; see the Degree symbol article for similar-appearing Unicode characters. In TeX, it is written \circ.