@article{DAN_1965_165_2_a0,
     author = {P. G. Aizengendler and M. M. Vainberg},
     title = {On periodic solutions of non-autonomous systems},
     journal = {Doklady Akademii Nauk},
     pages = {255--257},
     publisher = {mathdoc},
     volume = {165},
     number = {2},
     year = {1965},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1965_165_2_a0/}
}