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EXTENDING A RESULT OF RYAN ON WEAKLY COMPACT OPERATORS

Published online by Cambridge University Press:  30 May 2006

Kazuyuki Saitô
Affiliation:
Mathematical Institute, Tôhoku University, Aoba, Sendai 980-8578, Japan (yk.saito@beige.plala.or.jp)
J. D. Maitland Wright
Affiliation:
Mathematical Sciences, King’s College, University of Aberdeen, Aberdeen AB24 3UE, UK (jdmw@maths.abdn.ac.uk)
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Abstract

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An elegant result of Ryan gives a characterization of weakly compact operators from a Banach space $A$ into $c_{0}(X)$, the space of null sequences in a Banach space $X$. It would be a useful tool if the analogue of Ryan’s result were valid when $c_{0}(X)$ is replaced by $c(X)$, the space of convergent sequences in $X$. This seems plausible and has been assumed to be true by some authors. Unfortunately, it is false in general; Ylinen has produced a counterexample. But when $A$ is a $C^*$-algebra, or, more generally, when the dual of $A$ is weakly sequentially complete, we show that the desired extension of Ryan’s result does hold. The latter result turns out to be ‘best possible’.

Type
Research Article
Copyright
Copyright © Edinburgh Mathematical Society 2006