@article{AL_1971_10_6_a1,
     author = {Yu. L. Ershov},
     title = {Positive equivalences},
     journal = {Algebra i logika},
     pages = {620--650},
     publisher = {mathdoc},
     volume = {10},
     number = {6},
     year = {1971},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/AL_1971_10_6_a1/}
}