1. Aczel, M. J. (1949) Sur les operations defines pour nombres reels, Bull. Soc. Math. France, 76, 59?64.
2. Clifford, A. H. (1958a) Totally ordered commutative semigroups, Bull. Amer. Math. Soc. 64, 305?316.
3. Clifford, A. H. (1958b) Connected ordered topological semigroups with idempotent endpoints, Trans. Amer. Math. Soc. 88, 80?90.
4. Ling, C.-H. (1965) Representation of associative functions, Publ. Math. Debrecen 12, 189?212.
5. Narens, L. (1985) Abstract Measurement Theory, MIT Press, Cambridge, MA.