@article{DAN_1962_147_4_a15,
     author = {A. A. Fridman},
     title = {Degrees of unsolvability of the word problem for finitely presented groups},
     journal = {Doklady Akademii Nauk},
     pages = {805--808},
     publisher = {mathdoc},
     volume = {147},
     number = {4},
     year = {1962},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1962_147_4_a15/}
}