Chen Wu and  Lijuan Wang,  Invariant Characters of Tolerance Class and Reduction under Homomorphism in IIS.   journal   = {International Journal of Mathematical and Computational Sciences}, [online]. World Academy of Science, Engineering and Technology.
    January 2017, vol. 123(3). 135 - 139
    [viewed 21 April 2021]. Available from: https://publications.waset.org/pdf/10008418.