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Functional analysis is a branch of mathematical analysis which studies the transformations of functions and their algebraic and topological properties. The field builds upon and abstracts the results of Joseph Fourier's 1822 paper, Théorie analytique de la chaleur (The Analytical Theory of Heat), which demonstrated how a change of basis by means of the Fourier transform could be used to permit manipulations of a function in the frequency domain to obtain insights that were previously unobtainable. Functional analysis has modern applications in many areas of algebra, in particular associative algebra, in probability, operator theory, wavelets and wavelet transforms. The functional data analysis (FDA) paradigm of James O. Ramsay and Bernard Silverman ties functional analysis into principal component analysis and dimensionality reduction.
Functional analysis has strong parallels with linear algebra, as both fields are based on vector spaces as the core algebraic structure. Functional analysis endows linear algebra with concepts from topology (e.g. inner product, norm, topological space) in defining the topological vector space (TVS)[speculation?], which strengthens notions of continuity and limit, and supports generalization to infinite-dimensional spaces. The core operation within a TVS is linear transformation.
An important part of functional analysis is the extension of the theory of measure, integration, and probability to infinite dimensional spaces, also known as infinite dimensional analysis. Additionally, functional analysis generalizes the concept of an orthonormal basis--as found in Fourier analysis--to arbitrary inner product spaces, including those of infinite dimension. Important theoretical results include the Banach-Steinhaus theorem, spectral theorem (central to operator theory), Hahn-Banach theorem, open mapping theorem, and the closed graph theorem.
The historical roots of functional analysis lie in the study of spaces of functions and the formulation of properties of transformations of functions such as the Fourier transform as transformations defining continuous, unitary etc. operators between function spaces. This point of view turned out to be particularly useful for the study of differential and integral equations.
The usage of the word functional as a noun goes back to the calculus of variations, implying a function whose argument is a function. The term was first used in Leçons sur le calcul des variations (1910) by Jacques Hadamard. However, the general concept of a functional had previously been introduced in 1887 by the Italian mathematician and physicist Vito Volterra. The theory of nonlinear functionals was continued by students of Hadamard, in particular Maurice René Fréchet and Paul Lévy. Hadamard also founded the modern school of linear functional analysis further developed by Frigyes Riesz and the Polish Lwów School of Mathematics centered around Stefan Banach.
The basic and historically first class of spaces studied in functional analysis are complete normed vector spaces over the real or complex numbers. Such spaces are called Banach spaces. An important example is a Hilbert space, where the norm arises from an inner product. These spaces are of fundamental importance in many areas, including the mathematical formulation of quantum mechanics.
An important object of study in functional analysis are the continuous linear operators defined on Banach and Hilbert spaces. These lead naturally to the definition of C*-algebras and other operator algebras.
Hilbert spaces can be completely classified: there is a unique Hilbert space up to isomorphism for every cardinality of the orthonormal basis. Finite-dimensional Hilbert spaces are fully understood in linear algebra, and infinite-dimensional separable Hilbert spaces are isomorphic to . Separability being important for applications, functional analysis of Hilbert spaces consequently mostly deals with this space. One of the open problems in functional analysis is to prove that every bounded linear operator on a Hilbert space has a proper invariant subspace. Many special cases of this invariant subspace problem have already been proven.
General Banach spaces are more complicated than Hilbert spaces, and cannot be classified in such a simple manner as those. In particular, many Banach spaces lack a notion analogous to an orthonormal basis.
Examples of Banach spaces are -spaces for any real number . Given also a measure on set , then , sometimes also denoted or , has as its vectors equivalence classes of measurable functions whose absolute value's -th power has finite integral, that is, functions for which one has
If is the counting measure, then the integral may be replaced by a sum. That is, we require
Then it is not necessary to deal with equivalence classes, and the space is denoted , written more simply in the case when is the set of non-negative integers.
In Banach spaces, a large part of the study involves the dual space: the space of all continuous linear maps from the space into its underlying field, so-called functionals. A Banach space can be canonically identified with a subspace of its bidual, which is the dual of its dual space. The corresponding map is an isometry but in general not onto. A general Banach space and its bidual need not even be isometrically isomorphic in any way, contrary to the finite-dimensional situation. This is explained in the dual space article.
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Important results of functional analysis include:
The uniform boundedness principle or Banach-Steinhaus theorem is one of the fundamental results in functional analysis. Together with the Hahn-Banach theorem and the open mapping theorem, it is considered one of the cornerstones of the field. In its basic form, it asserts that for a family of continuous linear operators (and thus bounded operators) whose domain is a Banach space, pointwise boundedness is equivalent to uniform boundedness in operator norm.
There are many theorems known as the spectral theorem, but one in particular has many applications in functional analysis.
Theorem: Let A be a bounded self-adjoint operator on a Hilbert space H. Then there is a measure space and a real-valued essentially bounded measurable function f on X and a unitary operator such that
where T is the multiplication operator:
There is also an analogous spectral theorem for bounded normal operators on Hilbert spaces. The only difference in the conclusion is that now may be complex-valued.
The Hahn-Banach theorem is a central tool in functional analysis. It allows the extension of bounded linear functionals defined on a subspace of some vector space to the whole space, and it also shows that there are "enough" continuous linear functionals defined on every normed vector space to make the study of the dual space "interesting".
then there exists a linear extension ? : V -> R of ? to the whole space V, i.e., there exists a linear functional ? such that
The open mapping theorem, also known as the Banach-Schauder theorem (named after Stefan Banach and Juliusz Schauder), is a fundamental result which states that if a continuous linear operator between Banach spaces is surjective then it is an open map. More precisely,:
The proof uses the Baire category theorem, and completeness of both X and Y is essential to the theorem. The statement of the theorem is no longer true if either space is just assumed to be a normed space, but is true if X and Y are taken to be Fréchet spaces.
The closed graph theorem states the following: If X is a topological space and Y is a compact Hausdorff space, then the graph of a linear map T from X to Y is closed if and only if T is continuous.
Most spaces considered in functional analysis have infinite dimension. To show the existence of a vector space basis for such spaces may require Zorn's lemma. However, a somewhat different concept, Schauder basis, is usually more relevant in functional analysis. Many very important theorems require the Hahn-Banach theorem, usually proved using the axiom of choice, although the strictly weaker Boolean prime ideal theorem suffices. The Baire category theorem, needed to prove many important theorems, also requires a form of axiom of choice.
Functional analysis in its present formincludes the following tendencies: