WASET
    Ling Zhang and  Feng Liu,  Several Spectrally Non-Arbitrary Ray Patterns of Order 4.   journal   = {International Journal of Mathematical and Computational Sciences}, [online]. World Academy of Science, Engineering and Technology.
    November 2019, vol. 156(12). 233 - 236
    [viewed 25 April 2024]. Available from: https://publications.waset.org/pdf/10010962.