1. Fuzzy sets
2. Intuitionistic fuzzy sets
3. F. Smarandache, A unifying field in logics. neutrosophy: Neutrosophic probability, set and logic, American Research Press, Rehoboth (1999).
4. Pythagorean fuzzy subsets
5. C. Kahraman and F. K. Gündogdu, From 1D to 3D membership: spherical fuzzy sets (2018).