@article{DAN_1975_221_5_a5,
     author = {M. M. Lavrent'ev and V. R. Kireitov},
     title = {On branch points of optical hypersurfaces},
     journal = {Doklady Akademii Nauk},
     pages = {1027--1030},
     publisher = {mathdoc},
     volume = {221},
     number = {5},
     year = {1975},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1975_221_5_a5/}
}