1. P. Erdös, Problem 4358. Amer. Math. Monthly59, 480 (1949).
2. P.Erdös and R.Radò, Unpublished manuscript, referred to in [6] below.
3. L.Fuchs, Partially Ordered Algebraic Systems. Oxford 1963.
4. R.Gilmer, Commutative Semigroup Rings. Chicago 1984.
5. H. Hahn, Über die nichtarchimedischen Grö\ensysteme. Sitz. Akad. Wiss. Wien116, 601?655 (1907).