@article{DE_1987_23_2_a3,
     author = {A. I. Bulgakov},
     title = {On the oscillation of solutions of systems of second-order differential equations},
     journal = {Differencialʹnye uravneni\^a},
     pages = {204--217},
     publisher = {mathdoc},
     volume = {23},
     number = {2},
     year = {1987},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1987_23_2_a3/}
}