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2. A note on fuzzy sets;Brown;Information and Control,1971
3. Algebraic properties of fuzzy sets;De Luca;J. Math. Anal. Appl.,1972
4. L-fuzzy sets;Goguen;J. Math. Anal. Appl.,1967
5. Introduction à la Théorie des Sous-Ensembles Flous;Kaufmann,1973