@article{MAN_1981__256_163508,
     author = {Konrad Schm\"udgen},
     title = {Perturbations of {Selfadjoint} {Operators} with {Point} {Spectra} by {Restrictions} and {Selfadjoint} {Extensions.}},
     journal = {Mathematische Annalen},
     pages = {233--248},
     publisher = {mathdoc},
     volume = {256},
     year = {1981},
     zbl = {0443.47024},
     url = {http://geodesic.mathdoc.fr/item/MAN_1981__256_163508/}
}