@article{DAN_1981_256_4_a18,
     author = {A. M. Zamorzaev and A. F. Palistrant},
     title = {Geometric classification of the $P$-symmetries},
     journal = {Doklady Akademii Nauk},
     pages = {856--859},
     year = {1981},
     volume = {256},
     number = {4},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1981_256_4_a18/}
}