@article{MZM_1969_5_3_a8,
     author = {N. K. Nikol'skii},
     title = {On perturbations of the spectrum of unitary operators},
     journal = {Matemati\v{c}eskie zametki},
     pages = {341--349},
     publisher = {mathdoc},
     volume = {5},
     number = {3},
     year = {1969},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MZM_1969_5_3_a8/}
}