@article{INTM_1971_7_a0,
     author = {O. A. Oleinik and E. V. Radkevich},
     title = {Second order equations with nonnegative characteristic form},
     journal = {Itogi nauki i tehniki. Seri\^a, Matemati\v{c}eskij analiz},
     pages = {7--252},
     year = {1971},
     volume = {7},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/INTM_1971_7_a0/}
}