1. P. S. Novikov, ?On the algorithmic unsolvability of the word problem in group theory? Trudy Math. Inst., Acad. of Sci. USSR,44, 44 (1955).
2. Yu. V. Matiyasevich, ?Simple examples of unsolvable associative calculi? Dok. Akad. Nauk SSSR,173, No. 6, 1264?1266 (1967).
3. W. Bonne, ?The word problem,? Ann. of Math.,70, No. 2, 207?265 (1959).
4. J. L. Britton, ?The word problem,? Ann. of Math.,77, No. 1, 16?32 (1963).
5. G. Higman, B. H. Neumann, and H. Neumann, ?Embedding theorems for groups,? J. London Math. Soc.,24, 247?254 (1949).