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Functional analysis is the branch of mathematics, and specifically of analysis, concerned with the study of spaces of functions. It has its historical roots in the study of transformations, such as the Fourier transform, and in the study of differential and integral equations. This usage of the word functional goes back to the calculus of variations, implying a function whose argument is a function. Its use in general has been attributed to mathematician and physicist Vito Volterra and its founding is largely attributed to mathematician Stefan Banach.

Contents 1 Normed vector spaces 1.1 Hilbert spaces 1.2 Banach spaces 2 Major and foundational results 3 Foundations of mathematics considerations 4 Points of view 5 References

In the modern view, functional analysis is seen as the study of 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. More generally, functional analysis includes the study of Fréchet spaces and other topological vector spaces not endowed with a norm.

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 base. Since finite-dimensional Hilbert spaces are fully understood in linear algebra, and since morphisms of Hilbert spaces can always be divided into morphisms of spaces with Aleph-null (ℵ₀) dimensionality, functional analysis of Hilbert spaces mostly deals with the unique Hilbert space of dimensionality Aleph-null, and its morphisms. One of the open problems in functional analysis is to prove that every operator on a Hilbert space has a proper invariant subspace. Many special cases have already been proven.

General Banach spaces are more complicated. There is no clear definition of what would constitute a base, for example.

For any real number p ≥ 1, an example of a Banach space is given by "all Lebesgue-measurable functions whose absolute value's p-th power has finite integral" (see L^p spaces).

In Banach spaces, a large part of the study involves the dual space: the space of all continuous linear functionals. The dual of the dual is not always isomorphic to the original space, but there is always a natural monomorphism from a space into its dual's dual. This is explained in the dual space article.

Also, the notion of derivative can be extended to arbitrary functions between Banach spaces. See, for instance, the Fréchet derivative article.

Important results of functional analysis include:

• The uniform boundedness principle (also known as Banach-Steinhaus theorem) applies to sets of operators with tight bounds.
• One of the spectral theorems (there are indeed more than one) gives an integral formula for the normal operators on a Hilbert space. This theorem is of central importance for the mathematical formulation of quantum mechanics.
• The Hahn-Banach theorem extends functionals from a subspace to the full space, in a norm-preserving fashion. An implication is the non-triviality of dual spaces.
• The open mapping theorem and closed graph theorem.

See also: List of functional analysis topics.

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. Many very important theorems require the Hahn-Banach theorem, which relies on the axiom of choice that is strictly weaker than the Boolean prime ideal theorem.

Functional analysis in its present form includes the following tendencies:

• Soft analysis. An approach to analysis based on topological groups, topological rings, and topological vector spaces;
• Geometry of Banach spaces. A combinatorial approach primarily due to Jean Bourgain;
• Noncommutative geometry. Developed by Alain Connes, partly building on earlier notions, such as George Mackey's approach to ergodic theory;
• Connection with quantum mechanics. Either narrowly defined as in mathematical physics, or broadly interpreted by, e.g. Israel Gelfand, to include most types of representation theory.

• Brezis, H.: Analyse Fonctionnelle, Dunod ISBN 978-2100043149 or ISBN 978-2100493364
• Conway, John B.: A Course in Functional Analysis, 2nd edition, Springer-Verlag, 1994, ISBN 0-387-97245-5
• Dunford, N. and Schwartz, J.T. : Linear Operators, General Theory, and other 3 volumes, includes visualization charts
• Eidelman, Yuli, Vitali Milman, and Antonis Tsolomitis: Functional Analysis: An Introduction, American Mathematical Society, 2004.
• Hutson, V., Pym, J.S., Cloud M.J.: Applications of Functional Analysis and Operator Theory, 2nd edition, Elsevier Science, 2005, ISBN 0-444-51790-1
• Kolmogorov, A.N and Fomin, S.V.: Elements of the Theory of Functions and Functional Analysis, Dover Publications, 1999
• Kreyszig, Erwin: Introductory Functional Analysis with Applications, Wiley, 1989.
• Lax, P.: Functional Analysis, Wiley-Interscience, 2002
• Lebedev, L.P. and Vorovich, I.I.: Functional Analysis in Mechanics, Springer-Verlag, 2002
• Michel, Anthony N. and Charles J. Herget: Applied Algebra and Functional Analysis, Dover, 1993.
• Riesz, F. and Sz.-Nagy, B.: Functional Analysis, Dover Publications, 1990
• Rudin, W.: Functional Analysis, McGraw-Hill Science, 1991
• Schechter, M.: Principles of Functional Analysis, AMS, 2nd edition, 2001
• Shilov, Georgi E.: Elementary Functional Analysis, Dover, 1996.
• Sobolev, S.L.: Applications of Functional Analysis in Mathematical Physics, AMS, 1963
• Yosida, K.: Functional Analysis, Springer-Verlag, 6th edition, 1980

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