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.