Invariant subspaces of certain subnormal operators

AbstractLet T be a subnormal, nonnormal operator on a Hilbert space and suppose that the point spectrum of T∗ is empty. Then there exist vectors x ≠ 0 for which (T∗ − zI)−1x exists and is weakly continuous for all z. It is shown that under certain conditions, the Cauchy integral of this vector function taken around an appropriate contour, not necessarily lying in the resolvent set of T∗, leads to a proper (nontrivial) invariant subspace of T∗