@article{INTG_1971_3_a10,
     author = {V. V. Ryzhkov},
     title = {Characteristic directions of a~point mapping of $P_m$ into~$P_n$},
     journal = {Itogi Nauki i Tekhniki. Seriya Problemy Geometrii. Trudy Geometricheskogo Seminara},
     pages = {235--242},
     publisher = {mathdoc},
     volume = {3},
     year = {1971},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/INTG_1971_3_a10/}
}