1. S.N. Burris, H.P. Sankappanavar, A Course in Universal Algebra. Graduate Texts in Mathematics, vol. 78 (Springer-Verlag, Berlin, 1981)
2. S. Eilenberg, Automata, Languages and Machines, vol. B (Academic Press, New York, 1976)
3. Y.Q. Guo, H.J. Shyr, G. Thierrin, Qf-Disjunctive Languages, Papers on Automata and Languages, VII, K (Marx. Univ. Economics, Budapest, 1985), pp. 1–28
4. J.M. Howie, Fundamentals of Semigroup Theory (Oxford University Press, New York, 1995)
5. G. Lallement, Semigroups and Combinatorial Applications (Wiley Interscience, New York, 1979)