Invariant subspaces for operators whose spectra are Carathéodory regions

Abstract

In this paper it is shown that if an operator T satisfies ‖ p ( T ) ‖ ⩽ ‖ p ‖ σ ( T ) for every polynomial p and the polynomially convex hull of σ ( T ) is a Carathéodory region whose accessible boundary points lie in rectifiable Jordan arcs on its boundary, then T has a nontrivial invariant subspace. As a corollary, it is also shown that if T is a hyponormal operator and the outer boundary of σ ( T ) has at most finitely many prime ends corresponding to singular points on ∂ D and has a tangent at almost every point on each Jordan arc, then T has a nontrivial invariant subspace.