@article{DE_1986_22_9_a30,
     author = {P. A. Kuchment},
     title = {Periodic perturbations that preserve the law of dispersion of a differential operator},
     journal = {Differencialʹnye uravneni\^a},
     pages = {1641--1642},
     year = {1986},
     volume = {22},
     number = {9},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1986_22_9_a30/}
}