@article{DAN_1960_132_3_a13,
     author = {L. S. Pontryagin and L. V. Rodygin},
     title = {Periodic solution of a system of ordinary differential equations with a small parameter in the terms containing derivatives},
     journal = {Doklady Akademii Nauk},
     pages = {537--540},
     publisher = {mathdoc},
     volume = {132},
     number = {3},
     year = {1960},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1960_132_3_a13/}
}