A Banach space is a type of vector space, which is a space made up of vectors. In a Banach space, the length and distance between vectors can be calculated. It is also complete, meaning that a Cauchy sequence of vectors will always reach a limit. The concept was created and studied by Polish mathematician Stefan Banach in 1920-1922. The term "Banach space" was first used by Maurice René Fréchet, and Banach coined the term "Fréchet space". Banach spaces come from the study of function spaces by Hilbert, Fréchet, and Riesz earlier in the century, and play an important role in functional analysis.
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| Spaces | | | Properties |
- barrelled
- complete
- dual (algebraic/topological)
- locally convex
- reflexive
- separable
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| Theorems |
- Hahn–Banach
- Riesz representation
- closed graph
- uniform boundedness principle
- Kakutani fixed-point
- Krein–Milman
- min–max
- Gelfand–Naimark
- Banach–Alaoglu
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| Operators |
- adjoint
- bounded
- compact
- Hilbert–Schmidt
- normal
- nuclear
- trace class
- transpose
- unbounded
- unitary
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| Algebras |
- Banach algebra
- C*-algebra
- spectrum of a C*-algebra
- operator algebra
- group algebra of a locally compact group
- von Neumann algebra
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| Open problems |
- invariant subspace problem
- Mahler's conjecture
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| Applications |
- Hardy space
- spectral theory of ordinary differential equations
- heat kernel
- index theorem
- calculus of variations
- functional calculus
- integral operator
- Jones polynomial
- topological quantum field theory
- noncommutative geometry
- Riemann hypothesis
- distribution (or generalized functions)
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| Advanced topics |
- approximation property
- balanced set
- Choquet theory
- weak topology
- Banach–Mazur distance
- Tomita–Takesaki theory
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