@article{DE_1975_11_11_a4,
     author = {E. K. Ershov},
     title = {The stability of the rotation number of a doubly periodic differential equation},
     journal = {Differencialʹnye uravneni\^a},
     pages = {1949--1955},
     publisher = {mathdoc},
     volume = {11},
     number = {11},
     year = {1975},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1975_11_11_a4/}
}