@article{PIM_1998_N_S_63_77_a10,
     author = {M.A. Akivis and V.V. Goldberg},
     title = {Singular {Points} of {Lightlike} {Hypersurfaces} of the de {Sitter} {Space}},
     journal = {Publications de l'Institut Math\'ematique},
     pages = {81 },
     year = {1998},
     volume = {_N_S_63},
     number = {77},
     zbl = {0949.53015},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/PIM_1998_N_S_63_77_a10/}
}