Invariant subspaces for operators with spectrum containing the boundary of the numerical range

Abstract

We prove that a bounded Hilbert space operator has a nontrivial invariant subspace whenever its spectrum contains the topological boundary of its numerical range. We also give variants of this result and establish related statements. The criterion is applied to hyponormal, cohyponormal and Toeplitz operators. In these classes, convexity of the polynomial hull of the spectrum already suffices. Several examples are given that are not covered by well-known existence criteria.