1. Birkhoff, G.,Lattice Theory, AMS Colloq. Publ. Vol XXV, Providence, RI, 1967.
2. Brandon, R.L., D.W. Hardy and G. Markowsky,The Schützenberger Group of an H-class in the Semigroup of Binary Relations, Semigroup Forum, Vol. 5 (1972), 45–53.
3. Clifford, A.H. and G.B. Preston,The Algebraic Theory of Semigroups, AMS Surveys, Vol. I, Providence, RI, 1961.
4. Hopcroft, J.E. and J.D. Ullman,Formal Languages and Their Relation to Automata, Addison-Wesley, Reading, MA, 1969.
5. Landau, E.,Handbuch Verteilung der Primzahlen 1 1909, 222–229.