1. A.H. Clifford-G.B. Preston: The Algebraic theory of semigroups, I, Amer.Math.Soc., Providence, 1961.
2. Eqbal Ahmed-R. Wiegandt: On lower radicals of semigroups, Math.Nachr., 57 (1973), 163–167
3. R.S. Grigor: On the theory of radicals of semigroups, I,II, Mat.Issled, (Kišin'ov), 6 (1971), no. 4, 37–55 and 8 (1973), no. 4. 28–46 (in Russian).
4. H.-J. Hoehnke: Structure of semigroups, Canad.J.Math., 18 (1966), 449–491.
5. J. Kollar: Interpolation in semigroups, Semigroup Forum (to appear).