1. Burnell, D. G.,Left Cancellative Semigroups and an Embedding Problem, Doctoral Dissertation, Univ. of California, Davis (1970).
2. Chrislock, J. L.,The Structure of Archimedean Semigroups, Doctoral Dissertation, Univ. of California, Davis (1966).
3. Chrislock, J. L.,On Medial Semigroups, J. Algebra, 12 (1966), 1–9.
4. Clifford, A. H. and G. B. Preston,The Algebraic Theory of Semigroups, No. 7, Amer. Math. Soc., Providence, R. I. (1961).
5. Hall, R. E.,The Translational Hull of an N-Semigroup, Pacific J. Math., 41 (1972), 379–389.