Functional analysis
Functional analysis is a branch of
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 1910 book on that subject. However, the general concept of a functional had previously been introduced in 1887 by the Italian mathematician and physicist Vito Volterra.[1][2] The theory of nonlinear functionals was continued by students of Hadamard, in particular Fréchet and Lévy. 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 on functional analysis, the subject is seen as the study of vector spaces endowed with a topology, in particular infinite-dimensional spaces.[3][4] 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 theories of measure, integration, and probability to infinite dimensional spaces, also known as infinite dimensional analysis.
Normed vector spaces
The basic and historically first class of spaces studied in functional analysis are
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
Hilbert spaces
Hilbert spaces can be completely classified: there is a unique Hilbert space up to isomorphism for every cardinality of the orthonormal basis.[5] 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.
Banach spaces
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
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
Also, the notion of derivative can be extended to arbitrary functions between Banach spaces. See, for instance, the Fréchet derivative article.
Linear functional analysis
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Major and foundational results
There are four major theorems which are sometimes called the four pillars of functional analysis:
- the Hahn–Banach theorem
- the open mapping theorem
- the closed graph theorem
- the Banach–Steinhaus theorem.
Important results of functional analysis include:
Uniform boundedness principle
The
The theorem was first published in 1927 by Stefan Banach and Hugo Steinhaus but it was also proven independently by Hans Hahn.
Theorem (Uniform Boundedness Principle) — Let be a Banach space and be a normed vector space. Suppose that is a collection of continuous linear operators from to . If for all in one has
Spectral theorem
There are many theorems known as the spectral theorem, but one in particular has many applications in functional analysis.
Spectral theorem[7] — Let be a bounded self-adjoint operator on a Hilbert space . Then there is a measure space and a real-valued
This is the beginning of the vast research area of functional analysis called
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.
Hahn–Banach theorem
The
Hahn–Banach theorem:[8] — If is a sublinear function, and is a
Open mapping theorem
The
Open mapping theorem — If and are Banach spaces and is a surjective continuous linear operator, then is an open map (that is, if is an open set in , then is open in ).
The proof uses the Baire category theorem, and completeness of both and is essential to the theorem. The statement of the theorem is no longer true if either space is just assumed to be a
Closed graph theorem
The closed graph theorem states the following: If is a topological space and is a compact Hausdorff space, then the graph of a linear map from to is closed if and only if is
Other topics
Foundations of mathematics considerations
Most spaces considered in functional analysis have infinite dimension. To show the existence of a
Points of view
Functional analysis in its present form[update] includes the following tendencies:
- Abstract analysis. An approach to analysis based on topological groups, topological rings, and topological vector spaces.
- Geometry of combinatorial approach connected with Jean Bourgain; another is a characterization of Banach spaces in which various forms of the law of large numbershold.
- 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, for example, Israel Gelfand, to include most types of representation theory.
See also
References
- ^ Lawvere, F. William. "Volterra's functionals and covariant cohesion of space" (PDF). acsu.buffalo.edu. Proceedings of the May 1997 Meeting in Perugia. Archived (PDF) from the original on 2003-04-07.
- ISBN 978-93-86279-16-3.
- Springer Science & Business Media. p. 1.
- ^ Kadets, Vladimir (2018). A Course in Functional Analysis and Measure Theory [КУРС ФУНКЦИОНАЛЬНОГО АНАЛИЗА]. Springer. pp. xvi.
- OCLC 21228994.
- ^ Rynne, Bryan; Youngson, Martin A. Linear Functional Analysis. Retrieved December 30, 2023.
- ISBN 978-1-4614-7116-5.
- ^ ISBN 978-0-07-054236-5.
- ISBN 978-0-13-181629-9.
Further reading
- Aliprantis, C.D., Border, K.C.: Infinite Dimensional Analysis: A Hitchhiker's Guide, 3rd ed., Springer 2007, (by subscription)
- Bachman, G., Narici, L.: Functional analysis, Academic Press, 1966. (reprint Dover Publications)
- ISBN 0-444-70184-2
- ISBN 978-2-10-049336-4
- ISBN 0-387-97245-5
- Dunford, N. and Schwartz, J.T.: Linear Operators, General Theory, John Wiley & Sons, and other 3 volumes, includes visualization charts
- Edwards, R. E.: Functional Analysis, Theory and Applications, Hold, Rinehart and Winston, 1965.
- Eidelman, Yuli, Vitali Milman, and Antonis Tsolomitis: Functional Analysis: An Introduction, American Mathematical Society, 2004.
- Friedman, A.: Foundations of Modern Analysis, Dover Publications, Paperback Edition, July 21, 2010
- Giles, J.R.: Introduction to the Analysis of Normed Linear Spaces, Cambridge University Press, 2000
- Hirsch F., Lacombe G. - "Elements of Functional Analysis", Springer 1999.
- 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
- Kantorovitz, S.,Introduction to Modern Analysis, Oxford University Press, 2003,2nd ed.2006.
- Kolmogorov, A.N and Fomin, S.V.: Elements of the Theory of Functions and Functional Analysis, Dover Publications, 1999
- Kreyszig, E.: Introductory Functional Analysis with Applications, Wiley, 1989.
- ISBN 0-471-55604-1
- 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.
- Pietsch, Albrecht: History of Banach spaces and linear operators, Birkhäuser Boston Inc., 2007, ISBN 978-0-8176-4367-6
- Reed, M., Simon, B.: "Functional Analysis", Academic Press 1980.
- Riesz, F. and Sz.-Nagy, B.: Functional Analysis, Dover Publications, 1990
- Rudin, W.: Functional Analysis, McGraw-Hill Science, 1991
- Saxe, Karen: Beginning Functional Analysis, Springer, 2001
- 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
- Vogt, D., Meise, R.: Introduction to Functional Analysis, Oxford University Press, 1997.
- Yosida, K.: Functional Analysis, Springer-Verlag, 6th edition, 1980
External links
- "Functional analysis", Encyclopedia of Mathematics, EMS Press, 2001 [1994]
- Topics in Real and Functional Analysis by Gerald Teschl, University of Vienna.
- Lecture Notes on Functional Analysis by Yevgeny Vilensky, New York University.
- Lecture videos on functional analysis by Greg Morrow Archived 2017-04-01 at the Wayback Machine from University of Colorado Colorado Springs