Bhanu Gupta and  Sanjay K. Srivastava,  ψ-exponential Stability for Non-linear Impulsive Differential Equations.   journal   = {International Journal of Mathematical and Computational Sciences}, [online]. World Academy of Science, Engineering and Technology.
    August 2010, vol. 44(8). 1159 - 1162
    [viewed 21 February 2020]. Available from: https://publications.waset.org/pdf/5781.