Approximation property

File:MazurGes.jpg

The construction of a Banach space without the approximation property earned Per Enflo a live goose in 1972, which had been promised by Stanisław Mazur (left) in 1936.<ref>Megginson, Robert E. An Introduction to Banach Space Theory p. 336</ref>

In mathematics, specifically functional analysis, a Banach space is said to have the approximation property (AP), if every compact operator is a limit of finite-rank operators. The converse is always true.

Every Hilbert space has this property. There are, however, Banach spaces which do not; Per Enflo published the first counterexample in a 1973 article. However, much work in this area was done by Grothendieck (1955).

Later many other counterexamples were found. The space <math>\mathcal L(H)</math> of bounded operators on an infinite-dimensional Hilbert space <math>H</math> does not have the approximation property.<ref>Template:Cite journal</ref> The spaces <math>\ell^p</math> for <math>p\neq 2</math> and <math>c_0</math> (see Sequence space) have closed subspaces that do not have the approximation property.

Definition

A locally convex topological vector space X is said to have the approximation property, if the identity map can be approximated, uniformly on precompact sets, by continuous linear maps of finite rank.Template:Sfn

For a locally convex space X, the following are equivalent:Template:Sfn

1. X has the approximation property;
2. the closure of <math>X^{\prime} \otimes X</math> in <math>\operatorname{L}_p(X, X)</math> contains the identity map <math>\operatorname{Id} : X \to X</math>;
3. <math>X^{\prime} \otimes X</math> is dense in <math>\operatorname{L}_p(X, X)</math>;
4. for every locally convex space Y, <math>X^{\prime} \otimes Y</math> is dense in <math>\operatorname{L}_p(X, Y)</math>;
5. for every locally convex space Y, <math>Y^{\prime} \otimes X</math> is dense in <math>\operatorname{L}_p(Y, X)</math>;

where <math>\operatorname{L}_p(X, Y)</math> denotes the space of continuous linear operators from X to Y endowed with the topology of uniform convergence on pre-compact subsets of X.

If X is a Banach space this requirement becomes that for every compact set <math>K\subset X</math> and every <math>\varepsilon>0</math>, there is an operator <math>T\colon X\to X</math> of finite rank so that <math>\|Tx-x\|\leq\varepsilon</math>, for every <math>x \in K</math>.

Related definitions

Some other flavours of the AP are studied:

Let <math>X</math> be a Banach space and let <math>1\leq\lambda<\infty</math>. We say that X has the <math>\lambda</math>-approximation property (<math>\lambda</math>-AP), if, for every compact set <math>K\subset X</math> and every <math>\varepsilon>0</math>, there is an operator <math>T\colon X \to X</math> of finite rank so that <math>\|Tx - x\|\leq\varepsilon</math>, for every <math>x \in K</math>, and <math>\|T\|\leq\lambda</math>.

A Banach space is said to have bounded approximation property (BAP), if it has the <math>\lambda</math>-AP for some <math>\lambda</math>.

A Banach space is said to have metric approximation property (MAP), if it is 1-AP.

A Banach space is said to have compact approximation property (CAP), if in the definition of AP an operator of finite rank is replaced with a compact operator.

Examples

• Every subspace of an arbitrary product of Hilbert spaces possesses the approximation property.Template:Sfn In particular,
  • every Hilbert space has the approximation property.
  • every projective limit of Hilbert spaces, as well as any subspace of such a projective limit, possesses the approximation property.Template:Sfn
  • every nuclear space possesses the approximation property.
• Every separable Frechet space that contains a Schauder basis possesses the approximation property.Template:Sfn
• Every space with a Schauder basis has the AP (we can use the projections associated to the base as the <math>T</math>'s in the definition), thus many spaces with the AP can be found. For example, the <math>\ell^p</math> spaces, or the symmetric Tsirelson space.

Bibliography

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• Enflo, P.: A counterexample to the approximation property in Banach spaces. Acta Math. 130, 309–317(1973).
• Grothendieck, A.: Produits tensoriels topologiques et espaces nucleaires. Memo. Amer. Math. Soc. 16 (1955).
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• Paul R. Halmos, "Has progress in mathematics slowed down?" Amer. Math. Monthly 97 (1990), no. 7, 561—588. Template:MR
• William B. Johnson "Complementably universal separable Banach spaces" in Robert G. Bartle (ed.), 1980 Studies in functional analysis, Mathematical Association of America.
• Kwapień, S. "On Enflo's example of a Banach space without the approximation property". Séminaire Goulaouic–Schwartz 1972—1973: Équations aux dérivées partielles et analyse fonctionnelle, Exp. No. 8, 9 pp. Centre de Math., École Polytech., Paris, 1973. Template:MR
• Lindenstrauss, J.; Tzafriri, L.: Classical Banach Spaces I, Sequence spaces, 1977.
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• Karen Saxe, Beginning Functional Analysis, Undergraduate Texts in Mathematics, 2002 Springer-Verlag, New York.
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• Singer, Ivan. Bases in Banach spaces. II. Editura Academiei Republicii Socialiste România, Bucharest; Springer-Verlag, Berlin-New York, 1981. viii+880 pp. Template:ISBN. Template:MR