Singular Perturbations and Operators in Rigged Hilbert Spaces

A notion of regularity and singularity for a special class of operators acting in a rigged Hilbert space D⊂H⊂D× is proposed and it is shown that each operator decomposes into a sum of a regular and a singular part. This property is strictly related to the corresponding notion for sesquilinear forms. A particular attention is devoted to those operators that are neither regular nor singular, pointing out that a part of them can be seen as perturbation of a self-adjoint operator on H. Some properties for such operators are derived and some examples are discussed.