Root system of a perturbation of a selfadjoint operator with discrete spectrum (Q714631)

Let \(T\) be an unbounded self-adjoint operator having a discrete spectrum. The authors consider the perturbed operator \(T+B\), where \(B\) is a multiplication operator by a complex-valued function belonging to \(L^2(\mathbb{R})\). In particular, and under some conditions, the authors study the system of root vectors of \(T+B\). Other conditions allow them to construct a Riesz system of projections on invariant subspaces of \(T+B\).