WASET
    Zhaohao Wang and  Lan Shu,  The Lower and Upper Approximations in a Group.   journal   = {International Journal of Mathematical and Computational Sciences}, [online]. World Academy of Science, Engineering and Technology.
    August 2012, vol. 68(8). 1147 - 1151
    [viewed 20 April 2024]. Available from: https://publications.waset.org/pdf/9259.