@article{DE_1967_3_8_a18,
     author = {A. N. Vityuk},
     title = {On the existence of an upper and a lower solution of a boundary value problem for a system of ordinary differential equations},
     journal = {Differencialʹnye uravneni\^a},
     pages = {1393--1396},
     publisher = {mathdoc},
     volume = {3},
     number = {8},
     year = {1967},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1967_3_8_a18/}
}