Extensions of Ordinary Fuzzy Sets: A Comparative Literature Review

Abstract

Fuzzy sets extensions have been often used in the modeling of problems including vagueness and impreciseness in order to better define the membership functions together with the hesitancy of decision makers. More than 20 different extensions of ordinary fuzzy sets have appeared in the literature after the introductions of ordinary fuzzy sets by Zadeh (1965). These sets involve interval-type fuzzy sets, type-2 fuzzy sets, hesitant fuzzy sets, intuitionistic fuzzy sets, Pythagorean fuzzy sets, q-rung orthopair fuzzy sets, spherical fuzzy sets, picture fuzzy sets, fermatean fuzzy sets, etc. Mainly, these extensions can be divided into two classes: extensions with two independent membership parameters and extensions with three independent membership parameters. In this paper, we briefly classify these extensions and present some comparative graphical illustrations.