1. O. Andersen, ‘Ein Bericht über die Struktur abstrakter Halbgruppen’, Thesis (Staatsexamensarbeit), Hamburg, 1952.
2. S. Burris and H. P. Sankappanavar, A Course in Universal Algebra, Springer-Verlag, New York, 1981.
3. A. H. Clifford and G. B. Preston, ‘The Algebraic Theory of Semigroups’, Amer. Math. Soc., Providence, Vol. I, 1961, Vol. II, 1967.
4. P. M. Cohn, Universal Algebra, Reidel, Boston, 1981.
5. N. K. Dickson and P. R. Jones, ‘Finite groups and the basis property’, manuscript, 1976.