@article{DAN_1971_198_6_a4,
     author = {V. V. Laricheva},
     title = {Averaging for systems with variable period},
     journal = {Doklady Akademii Nauk},
     pages = {1269--1272},
     publisher = {mathdoc},
     volume = {198},
     number = {6},
     year = {1971},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1971_198_6_a4/}
}