1. S. I. Adian. The Burnside problem and identities in groups, volume 95 of Ergebnisse der Mathematik und ihrer Grenzgebiete [Results in Mathematics and Related Areas]. Springer-Verlag, Berlin-New York, 1979. Translated from the Russian by John Lennox and James Wiegold.
2. J. Brzozowski, K. Culik, and A. Gabrielian. Classification of non-counting events. J. Comp. Syst. Sci., 5:41–53, 1971.
3. Adyan, S. I. Problema Bernsaida i tozhdestva v gruppakh. Izdat. “Nauka”, Moscow, 1975.
4. Lecture Notes in Computer Science, 443;A. Luca de,1990
5. A. de Luca and S. Varricchio. On finitely recognizable semigroups. Acta Inform., 29(5):483–498, 1992.