Changjin Xu and  Peiluan Li,  Periodic Oscillations in a Delay Population Model.   journal   = {International Journal of Mathematical and Computational Sciences}, [online]. World Academy of Science, Engineering and Technology.
    August 2012, vol. 68(8). 1181 - 1184
    [viewed 22 October 2019]. Available from: https://publications.waset.org/pdf/576.