@article{DAN_1977_232_4_a7,
     author = {V. Ya. Ivrii},
     title = {The wave fronts of the solutions of the system that describes crystal optics},
     journal = {Doklady Akademii Nauk},
     pages = {763--765},
     publisher = {mathdoc},
     volume = {232},
     number = {4},
     year = {1977},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1977_232_4_a7/}
}