@article{DAN_1963_152_4_a7,
     author = {M. A. Krasnosel'skii},
     title = {An alternative principle for the existence of periodic solutions for differential equations with retarded argument},
     journal = {Doklady Akademii Nauk},
     pages = {801--804},
     publisher = {mathdoc},
     volume = {152},
     number = {4},
     year = {1963},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1963_152_4_a7/}
}