Changjin Xu and  Peiluan Li,  Periodic Orbits in a Delayed Nicholson's Blowflies Model.   journal   = {International Journal of Mathematical and Computational Sciences}, [online]. World Academy of Science, Engineering and Technology.
    September 2013, vol. 73(1). 99 - 102
    [viewed 01 April 2020]. Available from: https://publications.waset.org/pdf/16966.