Ruigang Zhang and  Wuyungaowa and  Xingchen Ma,  A Class of Formal Operators for Combinatorial Identities and its Application.   journal   = {International Journal of Mathematical and Computational Sciences}, [online]. World Academy of Science, Engineering and Technology.
    August 2013, vol. 75(3). 520 - 524
    [viewed 11 May 2021]. Available from: https://publications.waset.org/pdf/16808.