Maslina Darus and  Khalifa Al-Shaqsi,  Differential Sandwich Theorems with Generalised Derivative Operator.   journal   = {International Journal of Mathematical and Computational Sciences}, [online]. World Academy of Science, Engineering and Technology.
    February 2008, vol. 14(2). 149 - 152
    [viewed 02 December 2021]. Available from: https://publications.waset.org/pdf/13046.