Fuzzy ideals extensions of ordered semigroups. Xie, Xiang-Yun, Yan, Feng (2005) Lobachevskii Journal of Mathematics
Fuzzy structures of PI( ≪ , ⊆ , ⊆ ) BCK-ideals in hyper BCK-algebras. Jun, Young Bae, Shim, Wook Hwan (2003) International Journal of Mathematics and Mathematical Sciences
Fuzzy subcoalgebras and duality. Chen, Wenjuan (2009) Bulletin of the Malaysian Mathematical Sciences Society. Second Series