1. Mathematical Surveys, No. 7;Clifford, A. H.,1961
2. Semigroups satisfying variable identities;Putcha, Mohan S.;Semigroup Forum,1971
3. A semilattice decomposition into semigroups having at most one idempotent;Putcha, Mohan S.;Pacific J. Math.,1971
4. Semigroups satisfying identity 𝑥𝑦=𝑓(𝑥,𝑦);Tamura, Takayuki;Pacific J. Math.,1969
5. E. J. Tully, Semigroups satisfying an identity of the form 𝑥𝑦=𝑦^{𝑚}𝑥ⁿ (to appear).