1. A certain class of identities on semigroups
2. and , The algebraic theory of semigroups, Vol. I, Math. Surveys No. 7, Amer. Math. Soc., Providence, 1961.
3. Note on idempotent semigroups, I
4. Note on idempotent semigroups, III
5. Note on idempotent semigroups, IV, Identities of three variables