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@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/} }
A. A. Fridman. Degrees of unsolvability of the word problem for finitely presented groups. Doklady Akademii Nauk, Tome 147 (1962) no. 4, pp. 805-808. http://geodesic.mathdoc.fr/item/DAN_1962_147_4_a15/