@article{DE_1973_9_8_a22,
     author = {G. S. Rychkov},
     title = {A proof of the presence of an infinite number of limit cycles for the equation $\ddot y+\mu\ sin$ $(\dot y+\theta )+y=0$},
     journal = {Differencialʹnye uravneni\^a},
     pages = {1540--1542},
     publisher = {mathdoc},
     volume = {9},
     number = {8},
     year = {1973},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1973_9_8_a22/}
}