WASET
    Xun Ge and  Jinjin Li and  Ying Ge,  Some Separations in Covering Approximation Spaces.   journal   = {International Journal of Mathematical and Computational Sciences}, [online]. World Academy of Science, Engineering and Technology.
    July 2010, vol. 43(7). 1005 - 1009
    [viewed 23 April 2024]. Available from: https://publications.waset.org/pdf/13289.