1. Fuzzy class theory;Běhounek;Fuzzy Sets and Systems,2005
2. M. Black, Vagueness: An exercise in logical analysis, Philos. Sci. 4 (1937), 427–455. Reprinted in Int. J. Gen. Systems 17(1990), 107–128.
3. S. Gottwald, Universes of Fuzzy Sets—A Short Survey, Proc. 33rd International Symposium on Multiple-Valued Logic, May 2003, Tokyo, IEEE Comp. Soc.: Los Alamitos, 2003, pp. 71–76.
4. Metamathematics of Fuzzy Logic;Hájek,1998
5. A Development of Set Theory in Fuzzy Logic;Hájek,2003