Benshi Zhu,  Positive Solutions for Semipositone Discrete Eigenvalue Problems via Three Critical Points Theorem.   journal   = {International Journal of Mathematical and Computational Sciences}, [online]. World Academy of Science, Engineering and Technology.
    September 2010, vol. 45(9). 1257 - 1260
    [viewed 22 January 2020]. Available from: https://publications.waset.org/pdf/13982.