@article{DAN_1971_200_3_a18,
     author = {A. B. Shvartsburg},
     title = {Transition lines and singular points in nonlinear geometric optics equations},
     journal = {Doklady Akademii Nauk},
     pages = {575--578},
     publisher = {mathdoc},
     volume = {200},
     number = {3},
     year = {1971},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1971_200_3_a18/}
}