1. Zadeh, L.A.: Fuzzy sets. Inf. Control. 8(3), 338–353 (1965)
2. Atanassov, K.: Intuitionistic fuzzy sets. Fuzzy Sets Syst. 20(1), 87–96 (1986)
3. Zhang, W.R.: Bipolar fuzzy sets and relations: a computational framework for cognitive modeling and multiagent decision analysis. In: NAFIPS/IFIS/NASA'94. Proceedings of the First International Joint Conference of the North American Fuzzy Information Processing Society Biannual Conference. The Industrial Fuzzy Control and Intellige, pp. 305–309 (1994)
4. Tamir, D.E., Jin, L., Kandel, A.: A new interpretation of complex membership grade. Int. J. Intell. Syst. 26(4), 285–312 (2011)
5. Ramot, D., Milo, R., Friedman, M., Kandel, A.: Complex fuzzy sets. IEEE Trans. Fuzzy Syst. 10(2), 171–186 (2002)