@article{DAN_1961_137_4_a16,
     author = {B. M. Stepanov},
     title = {A formal definition of $R$-operations},
     journal = {Doklady Akademii Nauk},
     pages = {818--821},
     year = {1961},
     volume = {137},
     number = {4},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1961_137_4_a16/}
}