Yuli Hu and  Shaoquan Sun,  Fuzzy Subalgebras and Fuzzy Ideals of BCI-Algebras with Operators.   journal   = {International Journal of Mathematical and Computational Sciences}, [online]. World Academy of Science, Engineering and Technology.
    July 2017, vol. 126(6). 235 - 241
    [viewed 29 May 2022]. Available from: https://publications.waset.org/pdf/10007869.