1. Aîzenŝtat, A. Ja. and B.K. Boguta,On the lattice of varieties of semigroups, Semigr. Varieties and Semigr. of Endomorphisms, Leningrad (1979), 3–46 (Russian).
2. Birjukov, A.P.,Varieties of idempotent semigroups, Algebra i Logika 9(1970), 255–273 (Russian).
3. Clifford, A.H.,Semigroups admitting relative inverses, Ann. Math. 42 (1941), 1037–1049.
4. Clifford, A.H.,Bands of semigroups, Proc. Amer. Math. Soc. 5 (1954), 499–504.
5. Evans, T.,The lattice of semigroup varieties, Semigroup Forum 2(1971), 1–43.