1. Lesniewski, S.: Foundations of the general theory of sets (in Polish), Moscow, 1916; engl. transl. in:Surma, Srzednicki, Barnett, Rickey (eds.), Stanislaw Lesniewski, Collected Works, Kluwer, Dordrecht (1992).
2. Mamdani, E.H., Assilian, S.: An experiment in linguistic synthesis with a fuzzy logic controller, International Journal of Man-Machine Studies, 7 (1975) 1–13.
3. Pawlak, Z.: Rough Sets:Theoretical Aspects of Reasoning about Data, Kluwer, Dordrecht (1991).
4. Pawlak, Z., Skowron, A.: Rough membership functions in: Fedrizzi, M., Kacprzyk, J., Yager, R.R., eds., Advances in the Dempster-Shafer Theory of Evidence, Wiley, New York, (1994), 251–271.
5. Polkowski, L., Skowron, A.: Rough mereology, ICS Research Report 11/94, Institute of Computer Sci., Warsaw Univ. Technology, (1994).