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3. More on intuitionistic fuzzy sets;Atanassov;Fuzzy Sets and Systems,1989
4. Four new operators on intuitionistic fuzzy sets;Atanassov;Preprint IM-MFAIS-4-89,1989
5. A universal operator over intuitionistic fuzzy sets;Atanassov;Comptes Rendus de L'Academie Bulgare des Sciences,1993