Geodesic
On perturbations of the spectrum of unitary operators
Matematičeskie zametki, Tome 5 (1969) no. 3, pp. 341-349 
N. K. Nikol'skii. On perturbations of the spectrum of unitary operators. Matematičeskie zametki, Tome 5 (1969) no. 3, pp. 341-349. http://geodesic.mathdoc.fr/item/MZM_1969_5_3_a8/
@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},
     year = {1969},
     volume = {5},
     number = {3},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MZM_1969_5_3_a8/}
}
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We study the stability of the essential spectrum of unitary operators under nuclear and compact perturbations.