@article{DE_1972_8_10_a17,
     author = {N. G. Bulgakov},
     title = {On the existence of periodic solutions to nonlinear systems},
     journal = {Differencialʹnye uravneni\^a},
     pages = {1876--1877},
     publisher = {mathdoc},
     volume = {8},
     number = {10},
     year = {1972},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1972_8_10_a17/}
}