1. Arbib, M.A.,Algebraic theory of machines, languages, and semigroups, Academic Press, New York, 1968.
2. Cohn, P.M.,On sub-semigroups of free semigroups, Proc. Amer. Math. Soc. 13 (1962), 347–351.
3. Herman, G.T., and A. Walker,Context-free languages in biological systems, Dept. Rep. 60-73 (8/1/73). Dept. of Computer Sciences, SUNY at Buffalo, Buffalo, New York.
4. Krohn, K., and J. Rhodes,Algebraic theory of machines, Proc. Symp. on Automata Theory, Polytechnic Institute of Brooklyn (1962), 341–384.
5. Research Monograph;R. McNaughton,1971