@article{IVM_1974_12_a6,
     author = {R. M. Geidel'man},
     title = {Projective classification of ruled surfaces, and focal transforms of a~surface},
     journal = {Izvesti\^a vys\v{s}ih u\v{c}ebnyh zavedenij. Matematika},
     pages = {51--53},
     publisher = {mathdoc},
     number = {12},
     year = {1974},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/IVM_1974_12_a6/}
}