@article{MZM_1997_61_5_a14,
     author = {V. V. Filippov},
     title = {Existence of periodic solutions},
     journal = {Matemati\v{c}eskie zametki},
     pages = {769--784},
     publisher = {mathdoc},
     volume = {61},
     number = {5},
     year = {1997},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MZM_1997_61_5_a14/}
}