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  1. 2 Studia Mathematica

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  1. 2 Read C.

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  1. 1 2005

  2. 1 1999

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Normed "upper interval" algebras without nontrivial closed subalgebras

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Read C.
Studia Mathematica
|
2005
|
tom 171
|
nr 3
295-303
EN
It is a long standing open problem whether there is any infinite-dimensional commutative Banach algebra without nontrivial closed ideals. This is in some sense the Banach algebraists' counterpart to the invariant subspace problem for Banach spaces. We do not here solve this famous problem, but solve a related problem, that of finding (necessarily commutative) infinite-dimensional normed algebras which do not even have nontrivial closed subalgebras. Our examples are incomplete normed algebras rather than Banach algebras. The problem of finding such algebras, posed by W. Żelazko, was until now open not only for normed algebras but for more general topological algebras. Our construction here is kept short because it uses a key lemma involved in the construction of the "LRRW algebra" of Loy, Read, Runde and Willis. The algebras we find are dense subalgebras of certain commutative Banach algebras with compact multiplication.
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Strictly singular operators and the invariant subspace problem

100%
Read C.
Studia Mathematica
|
1999
|
tom 132
|
nr 3
203-226
EN
Properties of strictly singular operators have recently become of topical interest because the work of Gowers and Maurey in [GM1] and [GM2] gives (among many other brilliant and surprising results, such as those in [G1] and [G2]) Banach spaces on which every continuous operator is of form λ I + S, where S is strictly singular. So if strictly singular operators had invariant subspaces, such spaces would have the property that all operators on them had invariant subspaces. However, in this paper we exhibit examples of strictly singular operators without nontrivial closed invariant subspaces. So, though it may be true that operators on the spaces of Gowers and Maurey have invariant subspaces, yet this cannot be because of a general result about strictly singular operators. The general assertion about strictly singular operators is false.
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