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11 - On some operator ideals defined by approximation numbers

Published online by Cambridge University Press:  04 August 2010

Fernando Cobos
Affiliation:
Dpto. de Matemáticas, Univ. Autónoma de Madrid, Madrid, Spain
Ivam Resina
Affiliation:
Inst. de Matemática, Univ. Estadual de Campinas,S.Paulo, Brasil
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Summary

Abstract. We prove a representation theorem in terms of finite rank operators for operators belonging to. Some information on the tensor product of operators belonging to these ideals is also obtained.

INTRODUCTION.

The n-th approximation number an (T) of a bounded linear operator T∈ (E, F) acting between the Banach spaces E and F, is defined as

For the ideal is formed by all operators T betwen Banach space, with a finite quasi-norm

Weyl ideals are defined in a similar way, by substituting approximation numbers for Weyl numbers (xn (T)). Ideals have been studied by the authors in. Since xn (T) ≤ an (T) for al 1 n ∈ N (see), as a direct consequence of, Thm. 3, we have

Theorem 1. Let 0 < n ∞. Then there is a constant M = M such that for any complex Banach space E and any operator T ∈∞, ∞ (E, E) the following holds

Here (λn (T)) denotes the sequence of all eigenvalues of the compact operator T counted accoding to their algebraic multiplicities and ordered such that |λ1 (T)| ≥ |λ2(T)| ≥ … ≥ 0.

In this note we continue the study of -ideals. We derive a representation theorem for the elements of in terms of finite rank operators. This result is on the same lines as we established in for the case of the ideals (0 < q < ∞). We also obtain some information on the tensor pordect of operator belonging to the scale of the ideals.

Type
Chapter
Information
Geometric Aspects of Banach Spaces
Essays in Honour of Antonio Plans
, pp. 133 - 139
Publisher: Cambridge University Press
Print publication year: 1989

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