@article{DAN_1964_158_5_a9,
     author = {Karl-Peter Hadeler},
     title = {On the spectrum of normal operators and perturbations of them},
     journal = {Doklady Akademii Nauk},
     pages = {1042--1043},
     publisher = {mathdoc},
     volume = {158},
     number = {5},
     year = {1964},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1964_158_5_a9/}
}