Yiwan Guo and  Fahui Zhai,  Spherical Spectrum Properties of Quaternionic Operators.   journal   = {International Journal of Mathematical and Computational Sciences}, [online]. World Academy of Science, Engineering and Technology.
    January 2016, vol. 109(1). 15 - 18
    [viewed 27 April 2024]. Available from: https://publications.waset.org/pdf/10003650.