@article{DAN_1968_180_5_a0,
     author = {V. I. Amstislavskii},
     title = {Extension of recursive hierarchies, and $R$-operations},
     journal = {Doklady Akademii Nauk},
     pages = {1023--1026},
     publisher = {mathdoc},
     volume = {180},
     number = {5},
     year = {1968},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1968_180_5_a0/}
}