1. Etterbeck, W.,On N-semigroups, Thesis, University of California, Davis (1976).
2. Fuchs, L.,Partially Ordered Algebraic Systems, Pergamon Press 1963.
3. Tamura, T., Examples of completely exclusive direct product,Port.Math., 23(1960), 41–54.
4. Tamura, T., Construction of trees and commutative Archimedean semigroups,Math. Nachr. 36(1968), 257–287.
5. Tully, E. J., A class of naturally partially ordered commutative Archimedean semigroups with maximal condition,Proc. Amer. Math. Soc., 17(1960), 1133–1139.