Xiaojing Luo, Jingjing Song, Huili Dou, Xibei Yang, Taihua Xu. Hierarchies and Uncertainty Measures on Pythagorean Fuzzy Approximation Spaces. In Antonio J. Tallón-Ballesteros, editor, Fuzzy Systems and Data Mining VI - Proceedings of FSDM 2020, Virtual Event, November 13-16, 2020. Volume 331 of Frontiers in Artificial Intelligence and Applications, pages 643-655, IOS Press, 2020. [doi]