@article{AL_1968_7_1_a3,
     author = {Yu. L. Ershov},
     title = {A certain hierarchy of sets. {I}},
     journal = {Algebra i logika},
     pages = {47--74},
     publisher = {mathdoc},
     volume = {7},
     number = {1},
     year = {1968},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/AL_1968_7_1_a3/}
}