1. Zadeh, A. (1965). Fuzzy sets. Information and Control, 8(3), 338–353.
2. Atanassov, K. (1986). Intuitionistic fuzzy sets. Fuzzy Sets and Systems, 20(1), 87–96.
3. Smarandache, F. (1998). Neutrosophy. Neutrosophic probability, set, and logic. In ProQuest information & learning, Ann Arbor, Michigan, USA (p. 105).
4. Zhang, W. R. (1994). Bipolar fuzzy sets and relations: a computational frame work for cognitive modelling and multiagent decision analysis, Proceedings of the First International Joint Conference of The North American Fuzzy Information Processing Society Biannual Conference. The Industrial Fuzzy Control and Intelligence, 305–309.
5. Rosenfeld, A. (1975). Fuzzy graphs. In L. A. Zadeh, K. S. Fu, & M. Shimura (Eds.), Fuzzy sets and their applications (pp. 77–95). New York: Academic Press.