@article{DAN_1989_306_2_a6,
     author = {R. K. Romanovskii},
     title = {Averaging hyperbolic equations},
     journal = {Doklady Akademii Nauk},
     pages = {286--289},
     publisher = {mathdoc},
     volume = {306},
     number = {2},
     year = {1989},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1989_306_2_a6/}
}