@article{DE_1991_27_9_a4,
     author = {A. A. Boichuk and N. A. Perestyuk and A. M. Samoilenko},
     title = {Periodic solutions of impulse differential systems in critical cases},
     journal = {Differencialʹnye uravneni\^a},
     pages = {1516--1521},
     publisher = {mathdoc},
     volume = {27},
     number = {9},
     year = {1991},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1991_27_9_a4/}
}