1. Regular Algebra and Finite Machines;Conway,1971
2. S. Crvenković and R.Sz. Madarász, Some undecidability results, J. Symbolic Comput., in print.
3. Automata, Languages, and Machines, Vol. A;Eilenberg,1974
4. Algebraic Theory of Automata;Gécseg,1972
5. Universal Algebra;Grätzer,1979