1. Bulman-Fleming, S.: Flat and strongly flatS-systems. Comm. Algebra20(9), 2553?2567 (1992).
2. Bulman-Fleming, S., Normak, P.: Monoids over which all flat cyclic right acts are strongly flat. Semigroup Forum50, 233?241 (1995).
3. Cohn, P. M.: Universal Algebra. Dordrecht: D. Reidel. 1965.
4. Fleischer, V.: Completely flat monoids. Tartu Riikl. Ul. Toimetised610, 38?52 (1982), English translation: Amer. Math. Soc. Transl. (2)142, 19?31 (1989).
5. Kilp, M., Knauer, U.: Acts over monoids. (Unpublished manuscript, 1994).