1. Commutative cancellative semigroups without idempotents;Hamilton;Pacific J. Math.,1975
2. Homomorphisms on N-semigroups into R+ and the structure of N-semigroups;Kobayashi;J. Math. Tokushima Univ.,1973
3. Conditions for commutative semigroups to have nontrivial homomorphisms into nonnegative (positive) reals;Kobayashi,1977
4. Homomorphisms of commutative cancellative semigroups into nonnegative real numbers;Putcha;Trans. Amer. Math. Soc.,1976
5. R-congruences of R-semigroups;Tamura;J. Algebra,1973