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2. S. I. Adyan and V. G. Durnev, “Algorithmic problems for groups and semigroups,” Uspekhi Mat. Nauk 55(2), 3–94 (2000) [Russian Math. Surveys 55 (2), 207–296 (2000)].
3. O. A. Sarkisyan, Word Problem for Some Classes of Groups and Semigroups, Candidate’s Dissertation in Mathematics and Physics (MGU, Moscow, 1983) [in Russian].