Chapter 1: Hilbert space: geometry, projection on convex sets, orthogonal decomposition, bases, examples. Riesz representation theorem for bounded functionals. Spaces of sequences l p. Spaces of continuous functions C X. L p spaces.

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Functional analysis is a branch of mathematical analysis , the core of which is formed by the study of vector spaces endowed with some kind of limit-related structure e. 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.

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 Hadamard 's book on that subject. However, the general concept of a functional had previously been introduced in by the Italian mathematician and physicist Vito Volterra. Hadamard also founded the modern school of linear functional analysis further developed by Riesz and the group of Polish mathematicians around Stefan Banach.

In modern introductory texts to functional analysis, the subject is seen as the study of vector spaces endowed with a topology, in particular infinite-dimensional spaces. In contrast, linear algebra deals mostly with finite-dimensional spaces, and does not use topology. 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.

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. Hilbert spaces can be completely classified: there is a unique Hilbert space up to isomorphism for every cardinality of the orthonormal basis.

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. That is, we require. 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. Also, the notion of derivative can be extended to arbitrary functions between Banach spaces. 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. The theorem was first published in by Stefan Banach and Hugo Steinhaus but it was also proven independently by Hans Hahn.

Theorem Uniform Boundedness Principle. Let X be a Banach space and Y be a normed vector space. Suppose that F is a collection of continuous linear operators from X to Y. If for all x in X one has. There are many theorems known as the spectral theorem , but one in particular has many applications in functional analysis. Theorem: [4] Let A be a bounded self-adjoint operator on a Hilbert space H.

This is the beginning of the vast research area of functional analysis called operator theory ; see also the spectral measure. There is also an analogous spectral theorem for bounded normal operators on Hilbert spaces. 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".

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,: [5]. The proof uses the Baire category theorem , and completeness of both X and Y is essential to the theorem.

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 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 form [update] includes the following tendencies:. From Wikipedia, the free encyclopedia. For the assessment and treatment of human behavior, see Functional analysis psychology.

Main article: Banach-Steinhaus theorem. Main article: Spectral theorem. Main article: Hahn—Banach theorem. Main article: Open mapping theorem functional analysis.

Main article: Closed graph theorem. Main article: List of functional analysis topics. Functional analysis Dover ed. New York: Dover Publications. Functional analysis. Aliprantis, C. Online doi : Theory of Linear Operations. Friedman, A. Hutson, V. Kolmogorov, A. N and Fomin, S. Lax, P. Riesz, F. Sobolev, S. Yosida, K. Functional analysis at Wikipedia's sister projects.

Vector space Linear operators Functionals. Closed Compact operator Continuous and Discontinuous linear maps Densely defined. Bounding points Extreme point Interior.

Banach spaces. Banach and Normed spaces Norm Completeness. Auxiliary normed space. Hilbert spaces. Adjoint Inner product and Semi-inner-product Hilbert space and Prehilbert space. Bessel's inequality Cauchy—Schwarz inequality Riesz representation. Parseval's identity Polarization identity. Dirac spectrum Essential spectrum Pseudospectrum Structure space Shilov boundary. Abstract index group Banach algebra cohomology Cohen—Hewitt factorization theorem Extensions of symmetric operators Limiting absorption principle Unbounded operator.

Wiener algebra. Almost Mathieu operator Corona theorem Hearing the shape of a drum Dirichlet eigenvalue Heat kernel Kuznetsov trace formula Lax pair Proto-value function Ramanujan graph Rayleigh—Faber—Krahn inequality Spectral geometry Spectral method Spectral theory of ordinary differential equations Sturm—Liouville theory Superstrong approximation Transfer operator Transform theory Weyl law.

Duality and Spaces of Linear maps. Dual space Dual system Dual topology Duality Topology of uniform convergence. Banach—Alaoglu Mackey—Arens. Transpose of a linear map. Saturated family. Boundedness and Bornology. Barrelled space Bounded set Bornological space Ultrabornological space.

Un Bounded operator. Convex analysis. Mazur's lemma Robinson-Ursescu Simons Ursescu.

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## Functional analysis

Complete metric spaces. Fixed point theorem. Baire theorem. Compactness: equivalent definitions.

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