Yanling Zhu,  Periodic Solutions for a Higher Order Nonlinear Neutral Functional Differential Equation.   journal   = {International Journal of Mathematical and Computational Sciences}, [online]. World Academy of Science, Engineering and Technology.
    March 2011, vol. 51(3). 470 - 474
    [viewed 23 January 2020]. Available from: https://publications.waset.org/pdf/15240.