Xun Ge and  Zhaowen Li,  Definable Subsets in Covering Approximation Spaces.   journal   = {International Journal of Mathematical and Computational Sciences}, [online]. World Academy of Science, Engineering and Technology.
    March 2011, vol. 51(3). 475 - 478
    [viewed 26 April 2024]. Available from: https://publications.waset.org/pdf/7856.