@article{DAN_1957_117_2_a4,
     author = {M. A. Krasnosel'skii},
     title = {Periodical solutions in the neighbourhood of a singular point of a dynamical system},
     journal = {Doklady Akademii Nauk},
     pages = {180--183},
     publisher = {mathdoc},
     volume = {117},
     number = {2},
     year = {1957},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1957_117_2_a4/}
}