WASET
    Mei He and  Ying Ge and  Jingyu Qian,  On an Open Problem for Definable Subsets of Covering Approximation Spaces.   journal   = {International Journal of Mathematical and Computational Sciences}, [online]. World Academy of Science, Engineering and Technology.
    March 2011, vol. 51(3). 511 - 513
    [viewed 23 April 2024]. Available from: https://publications.waset.org/pdf/7038.