Li Xiguang,  Positive Solutions of Second-order Singular Differential Equations in Banach Space.   journal   = {International Journal of Mathematical and Computational Sciences}, [online]. World Academy of Science, Engineering and Technology.
    December 2011, vol. 60(12). 2089 - 2092
    [viewed 19 April 2024]. Available from: https://publications.waset.org/pdf/12457.