@article{DAN_1957_114_2_a4,
     author = {N. N. Krasovskii},
     title = {On periodical solutions of differential equations involving a time lag},
     journal = {Doklady Akademii Nauk},
     pages = {252--255},
     publisher = {mathdoc},
     volume = {114},
     number = {2},
     year = {1957},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1957_114_2_a4/}
}