1. Orthodox semigroups which are unions of groups;Clifford,1974
2. The Algebraic Theory of Semigroups;Clifford,1961
3. Perfect semigroups decomposable into a semilattice of rectangular groups (Russian);Fortunatov,1970
4. Almost commutative bands;Hall;Glasgow Math. J.,1972
5. Semigroups whose lattice of subsemigroups is complemented (Russian);Kacman;Sibirsk. Mat. Z.,1974