1. Adams D.H.,Semigroups with no nonzero nilpotent elements, Math. Z., 123 (1971), 168–176.
2. Bosak J.,On radicals of semigroups, Mat.-fyz. časopis, 12 (1962), 230–234.
3. Bosak J.,On radicals of semigroups, Mat. časopis, 18 (1968), 204–212.
4. Chick H., B.J. Gardner,The preservation of some ring properties by semilattice sums, Commun. Algebra, 15 (1987), 1017–1038.
5. Clifford A., G. Preston,The algebraic theory of semigroups, 1964.