@article{IVM_1957_1_a1,
     author = {M. A. Akivis},
     title = {Focal maps of a~surface of rank~$r$},
     journal = {Izvesti\^a vys\v{s}ih u\v{c}ebnyh zavedenij. Matematika},
     pages = {9--19},
     publisher = {mathdoc},
     number = {1},
     year = {1957},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/IVM_1957_1_a1/}
}