@article{IVM_1977_3_a2,
     author = {V. A. Zalgaller and F. L. Litvin},
     title = {A sufficient criterion for the existence of an envelope of characteristics and a~cuspidal edge on the enveloping surface of a~parametrized family of surfaces},
     journal = {Izvesti\^a vys\v{s}ih u\v{c}ebnyh zavedenij. Matematika},
     pages = {20--23},
     publisher = {mathdoc},
     number = {3},
     year = {1977},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/IVM_1977_3_a2/}
}