Compactness (mathematics) | Operator theory

Compact operator

In functional analysis, a branch of mathematics, a compact operator is a linear operator , where are normed vector spaces, with the property that maps bounded subsets of to relatively compact subsets of (subsets with compact closure in ). Such an operator is necessarily a bounded operator, and so continuous. Some authors require that are Banach, but the definition can be extended to more general spaces. Any bounded operator that has finite rank is a compact operator; indeed, the class of compact operators is a natural generalization of the class of finite-rank operators in an infinite-dimensional setting. When is a Hilbert space, it is true that any compact operator is a limit of finite-rank operators, so that the class of compact operators can be defined alternatively as the closure of the set of finite-rank operators in the norm topology. Whether this was true in general for Banach spaces (the approximation property) was an unsolved question for many years; in 1973 Per Enflo gave a counter-example, building on work by Grothendieck and Banach. The origin of the theory of compact operators is in the theory of integral equations, where integral operators supply concrete examples of such operators. A typical Fredholm integral equation gives rise to a compact operator K on function spaces; the compactness property is shown by equicontinuity. The method of approximation by finite-rank operators is basic in the numerical solution of such equations. The abstract idea of Fredholm operator is derived from this connection. (Wikipedia).

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Properties of Compactness

Compact sets enjoy some mysterious properties, which I'll discuss in this video. More precisely, compact sets are always bounded and closed. The beauty of this result lies in the proof, which is an elegant application of this subtle concept. Enjoy! Compactness Definition: https://youtu.be

From playlist Topology

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From playlist PHYSICS 67.1 ADVANCED E&M VECTORS & FIELDS

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From playlist Abstract algebra

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From playlist Abstract algebra

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From playlist Course 7: (Rudin's) Principles of Mathematical Analysis

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From playlist Global Noncommutative Geometry Seminar (Europe)

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From playlist MIT 18.102 Introduction to Functional Analysis, Spring 2021

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From playlist Mathematics

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From playlist MIT 18.102 Introduction to Functional Analysis, Spring 2021

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From playlist Global Noncommutative Geometry Seminar (Europe)

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From playlist Mathematics

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From playlist MIT 18.102 Introduction to Functional Analysis, Spring 2021

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Compactness

The single, most important concept in topology and analysis: Compactness. This is explained via covers, which I'll define as well. There are tons of applications of this concept, which you can find in the playlist below Topology Playlist: https://youtube.com/playlist?list=PLJb1qAQIrmmA13v

From playlist Topology

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Elliptic boundary value problem | Countable set | Functional analysis | Operator norm | Closure (topology) | Fredholm alternative | Compact embedding | Separable space | Ideal (ring theory) | Normed vector space | Riesz's lemma | Fredholm operator | Frigyes Riesz | Adjoint | Alexander Grothendieck | Operator ideal | Sobolev space | Topological vector space | Bounded operator | Algebra over a field | Singular value decomposition | Lax–Milgram theorem | Neighbourhood (mathematics) | Stefan Banach | Bounded set | Mathematics | Spectrum (functional analysis) | Totally bounded space | Calkin algebra | Function space | Equicontinuity | Trace class | Integral equation | Hilbert space | Weak topology | Hilbert–Schmidt integral operator | Transpose | Kernel (algebra) | Finite-rank operator | Approximation property | Fredholm integral equation | Nuclear operator | Spectral theory of compact operators