Operator (mathematics)

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This article is about operators in mathematics. For other uses, see Operator (disambiguation).
Not to be confused with the symbol denoting a mathematical operation or mathematical symbol.

In mathematics, an operator is generally a mapping that acts on the elements of a space to produce other elements of the same space. The most common operators are linear maps, which act on vector spaces. However, when using "linear operator" instead of "linear map", mathematicians mean often actions on vector spaces of functions, which preserve also other properties, such as continuity. For example, differentiation and indefinite integration are linear operators; operators that are built from them are called differential operators, integral operators or integro-differential operators.

Operator is also used for denoting the symbol of a mathematical operation. This is related with the meaning of "operator" in computer programming, see operator (computer programming).

Linear operators[edit]

Main article: Linear operator

The most common kind of operator encountered are linear operators. Let U and V be vector spaces over a field K. Operator A: U → V is called linear if

for all x, y in U and for all α, β in K.

The importance of linear operators is partially because they are morphisms between vector spaces.

In finite-dimensional case linear operators can be represented by matrices in the following way. Let be a field, and and be finite-dimensional vector spaces over . Let us select a basis in and in . Then let be an arbitrary vector in (assuming Einstein convention), and be a linear operator. Then


Then is the matrix of the operator in fixed bases. does not depend on the choice of , and iff . Thus in fixed bases n-by-m matrices are in bijective correspondence to linear operators from to .

The important concepts directly related to operators between finite-dimensional vector spaces are the ones of rank, determinant, inverse operator, and eigenspace.

Linear operators also play a great role in the infinite-dimensional case. The concepts of rank and determinant cannot be extended to infinite-dimensional matrices. This is why very different techniques are employed when studying linear operators (and operators in general) in the infinite-dimensional case. The study of linear operators in the infinite-dimensional case is known as functional analysis (so called because various classes of functions form interesting examples of infinite-dimensional vector spaces).

The space of sequences of real numbers, or more generally sequences of vectors in any vector space, themselves form an infinite-dimensional vector space. The most important cases are sequences of real or complex numbers, and these spaces, together with linear subspaces, are known as sequence spaces. Operators on these spaces are known as sequence transformations.

Bounded linear operators over Banach space form a Banach algebra in respect to the standard operator norm. The theory of Banach algebras develops a very general concept of spectra that elegantly generalizes the theory of eigenspaces.

Bounded operators[edit]

Let U and V be two vector spaces over the same ordered field (for example, ), and they are equipped with norms. Then a linear operator from U to V is called bounded if there exists C > 0 such that

for all x in U.

Bounded operators form a vector space. On this vector space we can introduce a norm that is compatible with the norms of U and V:


In case of operators from U to itself it can be shown that


Any unital normed algebra with this property is called a Banach algebra. It is possible to generalize spectral theory to such algebras. C*-algebras, which are Banach algebras with some additional structure, play an important role in quantum mechanics.



Main articles: general linear group and isometry

In geometry, additional structures on vector spaces are sometimes studied. Operators that map such vector spaces to themselves bijectively are very useful in these studies, they naturally form groups by composition.

For example, bijective operators preserving the structure of a vector space are precisely the invertible linear operators. They form the general linear group under composition. They do not form a vector space under the addition of operators, e.g. both id and -id are invertible (bijective), but their sum, 0, is not.

Operators preserving the Euclidean metric on such a space form the isometry group, and those that fix the origin form a subgroup known as the orthogonal group. Operators in the orthogonal group that also preserve the orientation of vector tuples form the special orthogonal group, or the group of rotations.

Probability theory[edit]

Main article: Probability theory

Operators are also involved in probability theory, such as expectation, variance, covariance, factorials, etc.


From the point of view of functional analysis, calculus is the study of two linear operators: the differential operator , and the indefinite integral operator .

Fourier series and Fourier transform[edit]

The Fourier transform is useful in applied mathematics, particularly physics and signal processing. It is another integral operator; it is useful mainly because it converts a function on one (temporal) domain to a function on another (frequency) domain, in a way effectively invertible. No information is lost, as there is an inverse transform operator. In the simple case of periodic functions, this result is based on the theorem that any continuous periodic function can be represented as the sum of a series of sine waves and cosine waves:

The tuple (a0, a1, b1, a2, b2, ...) is in fact an element of an infinite-dimensional vector space 2, and thus Fourier series is a linear operator.

When dealing with general function RC, the transform takes on an integral form:

Laplace transform[edit]

Main article: Laplace transform

The Laplace transform is another integral operator and is involved in simplifying the process of solving differential equations.

Given f = f(s), it is defined by:

Fundamental operators on scalar and vector fields[edit]

Three operators are key to vector calculus:

As an extension of vector calculus operators to physics, engineering and tensor spaces, Grad, Div and Curl operators also are often associated with Tensor calculus as well as vector calculus.[1]

See also[edit]


  1. ^ h.m. schey (2005). Div Grad Cural and All that. New York: W W Norton. ISBN 0-393-92516-1.