1. Birkhoff, G.,Lattice Theory, Vol. 25. AMS Colloquium Publications, Providence, 1967.
2. Clifford, A. H.,Semigroups admitting relative inverses. Annals of Math.42 (1941), 1037?1049.
3. Halmos, P. R.,Lectures on Boolean Algebras. Van Nostrand, New York, 1963.
4. Howie, J. M.,An Introduction to Semigroup Theory. Academic Press, New York, 1976.
5. Kimura, N.,Note on idempotent semigroups, I. Proc. Japan Acad.33 (1957), 642?645.