@article{AL_1976_15_6_a3,
     author = {Yu. L. Ershov},
     title = {Hereditarily effective operations},
     journal = {Algebra i logika},
     pages = {642--654},
     publisher = {mathdoc},
     volume = {15},
     number = {6},
     year = {1976},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/AL_1976_15_6_a3/}
}