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U-ideals of factorable operators

Kamil John (1999)

Czechoslovak Mathematical Journal

We suggest a method of renorming of spaces of operators which are suitably approximable by sequences of operators from a given class. Further we generalize J. Johnsons’s construction of ideals of compact operators in the space of bounded operators and observe e.g. that under our renormings compact operators are u -ideals in the: space of 2-absolutely summing operators or in the space of operators factorable through a Hilbert space.

Uncomplementability of spaces of compact operators in larger spaces of operators

Giovanni Emmanuele, Kamil John (1997)

Czechoslovak Mathematical Journal

In the first part of the paper we prove some new result improving all those already known about the equivalence of the nonexistence of a projection (of any norm) onto the space of compact operators and the containment of c 0 in the same space of compact operators. Then we show several results implying that the space of compact operators is uncomplemented by norm one projections in larger spaces of operators. The paper ends with a list of questions naturally rising from old results and the results...

Uncomplemented copies of C(K) inside C(K).

Francisco Arranz (1996)

Extracta Mathematicae

Throughout this note, whenever K is a compact space C(K) denotes the Banach space of continuous functions on K endowed with the sup norm. Though it is well known that every infinite dimensional Banach space contains uncomplemented subspaces, things may be different when only C(K) spaces are considered. For instance, every copy of l∞ = C(BN) is complemented wherever it is found. In [5] Pelzcynski found: Theorem 1. Let K be a compact metric space. If a separable Banach space X contains a subspace...

Uniqueness of unconditional bases in c 0 -products

P. Casazza, N. Kalton (1999)

Studia Mathematica

We give counterexamples to a conjecture of Bourgain, Casazza, Lindenstrauss and Tzafriri that if X has a unique unconditional basis (up to permutation) then so does c 0 ( X ) . We also give some positive results including a simpler proof that c 0 ( 1 ) has a unique unconditional basis and a proof that c 0 ( p n N n ) has a unique unconditional basis when p n 1 , N n + 1 2 N n and ( p n - p n + 1 ) l o g N n remains bounded.

Uniqueness of unconditional bases of c 0 ( l p ) , 0 < p < 1

C. Leránoz (1992)

Studia Mathematica

We prove that if 0 < p < 1 then a normalized unconditional basis of a complemented subspace of c 0 ( l p ) must be equivalent to a permutation of a subset of the canonical unit vector basis of c 0 ( l p ) . In particular, c 0 ( l p ) has unique unconditional basis up to permutation. Bourgain, Casazza, Lindenstrauss, and Tzafriri have previously proved the same result for c 0 ( l ) .

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