On some classes of unbounded operators
The authors have investigated the notions of (formally) hyponormal and (formally) normal operators for unbounded operators and discussed the quasi-similarity of such operators. In particular, they have shown that quasi-similar unbounded normal operators are unitarily equivalent (which is an unbounded generalization of the Douglas theorem for the bounded normal operators) and a closed subnormal operator quasi-similar to a self-adjoint operator is self- adjoint.
quasi-similar unbounded normal operators
unbounded generalization of the Douglas theorem for the bounded normal operators
closed subnormal operator quasi-similar to a self-adjoint operator is self-adjoint
scientific article; zbMATH DE number 4119046