Li Ge,  Existence of Solution for Four-Point Boundary Value Problems of Second-Order Impulsive Differential Equations (III).   journal   = {International Journal of Mathematical and Computational Sciences}, [online]. World Academy of Science, Engineering and Technology.
    April 2011, vol. 52(4). 707 - 711
    [viewed 20 January 2020]. Available from: https://publications.waset.org/pdf/11223.