@article{DE_1971_7_3_a18,
     author = {B. S. Kalitin},
     title = {The limit cycles of pendulum systems with impulse perturbation},
     journal = {Differencialʹnye uravneni\^a},
     pages = {540--542},
     publisher = {mathdoc},
     volume = {7},
     number = {3},
     year = {1971},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1971_7_3_a18/}
}