© 1986 by London Mathematical Society
A Short Proof Concerning the Invariant Subspace Problem
Department of Pure Mathematics and Mathematical Statistics 16 Mill Lane, Cambridge CB2 1SB
Counterexamples for the invariant subspace problem on a general Banach space exist due to Enflo [1], Read [3], Beauzamy [3] (simplification of [1]). On the space l1, there is a counterexample due to Read [4], which is rather long since it uses all of [3]. Here we present a short, direct proof that there is an invariant subspace free operator T on l1, and we also establish the following facts about our operator. First, it is a perturbation of a weighted shift operator by a nuclear operator. Secondly, its spectrum is identical with its approximate point spectrum, which is the unit ball of C. Thirdly, we can arrange either that no positive power of T has invariant subspaces, or alternately that every positive power T* except T itself has non-trivial invariant subspaces.