Yongkun Li and  Erliang Xu,  Multiple Positive Periodic Solutions of a Competitor-Competitor-Mutualist Lotka-Volterra System with Harvesting Terms.   journal   = {International Journal of Mathematical and Computational Sciences}, [online]. World Academy of Science, Engineering and Technology.
    January 2010, vol. 37(1). 55 - 60
    [viewed 24 January 2020]. Available from: https://publications.waset.org/pdf/12274.