Eberlein–Šmulian theorem

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In the mathematical field of functional analysis, the Eberlein–Šmulian theorem (named after William Frederick Eberlein and Witold Lwowitsch Schmulian) is a result that relates three different kinds of weak compactness in a Banach space.

Statement of the theorem

Types of weak compactness

A set A can be weakly compact in three different ways:

Compactness (or Heine-Borel compactness): Every open cover of A admits a finite subcover.
Sequential compactness: Every sequence from A has a convergent subsequence whose limit is in A.
Limit point compactness: Every infinite subset of A has a limit point in A.

The Eberlein–Šmulian theorem

The Eberlein–Šmulian theorem states that the three are equivalent on a Banach space. While this equivalence is true in general for a metric space, the weak topology is not metrizable in infinite dimensional vector spaces, and so the Eberlein–Šmulian theorem is needed.

Applications

The Eberlein–Šmulian theorem is important in the theory of PDEs, and particularly in Sobolev spaces. Many Sobolev spaces are reflexive Banach spaces and therefore bounded subsets are weakly precompact by Alaoglu's theorem. Thus the theorem implies that bounded subsets are weakly precompact, and therefore have limits in the weak sense. Since most PDEs only have solutions in the weak sense, this theorem is an important step in deciding which spaces of weak solutions to use in solving a PDE.

References

Diestel, Joseph (1984), Sequences and series in Banach spaces, Springer-Verlag, ISBN 0-387-90859-5 .
Dunford, N.; Schwartz, J.T. (1958), Linear operators, Part I, Wiley-Interscience .
Whitley, R.J. (1967), "An elementary proof of the Eberlein-Smulian theorem", Mathematische Annalen 172 (2): 116–118, doi:10.1007/BF01350091 .

v t e Functional analysis

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Important Theorems	Banach–Alaoglu Banach–Saks Banach–Mazur Bessel's inequality Cauchy–Schwarz inequality Closed range Closed graph Eberlein–Šmulian Freudenthal spectral Gelfand–Mazur Goldstine Hahn–Banach (Hyperplane separation) Kakutani fixed-point Lomonosov's Invariant Subspace Mackey–Arens Mazur's lemma M. Riesz extension Open mapping Parseval's identity Schauder fixed-point

Analysis	Bochner space Derivatives (Fréchet derivative, Gâteaux derivative, Functional, Holomorphy) Differentiation in Fréchet spaces Integrals (Bochner, Dunford, Gelfand–Pettis, Regulated, Weak) Functional calculus Inverse function theorem (Nash–Moser theorem) Vector measure

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