1. J. L. Chrislock, The structure of archimedean semigroups, Dissertation, University of California, Davis, Calif., 1966.
2. On medial semigroups;Chrislock, J. L.;J. Algebra,1969
3. Mathematical Surveys, No. 7;Clifford, A. H.,1961
4. R. P. Dickinson, Right zero unions, Dissertation, University of California, Davis, Calif., 1970.
5. R. E. Hall, The structure of certain commutative separative and commutative cancellative semigroups, Dissertation, Pennsylvania State University, Middletown, Pa., 1969.