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2. J. Łukasiewicz, Elementy logiki matematycznej, PWN, Warszawa, 1929 (Pergamon Press, Oxford, 1963). English translation Elements of Mathematical Logic
3. L.A. Zadeh, Fuzzy sets. Inf. Control 8, 338–353 (1967)
4. J. Kacprzyk, Fuzzy Sets in System Analysis (PWN, Warsaw, 1986)
5. J. Kacprzyk, Multistage Fuzzy Control: A Model-Based Approach to Fuzzy Control and Decision Making (Wiley, Inc, 1997)