1. Dickinson, R.P.,Right zero union of semigroups, Dissertation, University of California, Davis 1970.
2. Gratzer, G.,Universal algebra, Princeton, N.J., Van Nostrand.
3. Kimura, N.,The structure of idempotent semigroups (1), Pacific J. Math. 8 (1958), 257–275.
4. Kobayashi, Y.,Homomorphisms on N-semigroups into R+ and the structure of N-semigroups, Jour. Math. Tokushima University 7 (1973), 1–20.
5. Petrich, M.,Introduction to Semigroups, Charles E. Merrill Publishing Company, 1973.