1. Algebraic Theory of Lattices;Crawley,1973
2. Semimodularity in lattices of congruences;Eberhart;J. Algebra,1978
3. An Introduction to Semigroup Theory;Howie,1976
4. Congruences contained in an equivalence on a semigroup;Jones;J. Austral. Math. Soc. (Ser. A),1980
5. Introduction to Semigroups;Petrich,1973