@article{MZM_1988_44_4_a4,
     author = {V. A. Kondrat'ev and E. M. Landis},
     title = {Semilinear equations of second order with nonnegative characteristic form},
     journal = {Matemati\v{c}eskie zametki},
     pages = {457--468},
     publisher = {mathdoc},
     volume = {44},
     number = {4},
     year = {1988},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MZM_1988_44_4_a4/}
}