Weakly special and special radicals in semigroups

Hoehnke radicals on the class of semigroups are considered. The concepts of the special and weakly special radical in semigroups are introduced; they are analogous to the ring concepts. A characterization of reductive semigroups as those which possess a universal property for extensions is found. It is proved that the least special and least weakly special radicals coincide. A connection between the least special radical and the lower radical of Hoehnke is revealed.
Bibliography: 15 titles.