@article{MZM_1986_39_4_a9,
     author = {I. T. Kiguradze},
     title = {Periodic solutions of systems of nonautonomous ordinary differential equations},
     journal = {Matemati\v{c}eskie zametki},
     pages = {562--575},
     publisher = {mathdoc},
     volume = {39},
     number = {4},
     year = {1986},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MZM_1986_39_4_a9/}
}