The singularly continuous spectrum and non-closed invariant subspaces

Abstract

Let $\mathbf{A}$ be a bounded self-adjoint operator on a separable Hilbert space $\mathfrak{H}$ and ${\mathfrak{H}}_0\subset \mathfrak{H}$ a closed invariant subspace of $\mathbf{A}$. Assuming that ${\mathfrak{H}}_0$ is of codimension 1, we study the variation of the invariant subspace ${\mathfrak{H}}_0$ under bounded self-adjoint perturbations $\mathbf{V}$ of $\mathbf{A}$ that are off-diagonal with respect to the decomposition $\mathfrak{H}={\mathfrak{H}}_0\oplus {\mathfrak{H}}_1$. In particular, we prove the existence of a one-parameter family of dense non-closed invariant subspaces of the operator $\mathbf{A}+\mathbf{V}$ provided that this operator has a nonempty singularly continuous spectrum. We show that such subspaces are related to non-closable densely defined solutions of the operator Riccati equation associated with generalized eigenfunctions corresponding to the singularly continuous spectrum of $\mathbf{B}$.