Pandey, R., C. Shukla, A. Shukla, A. Upadhyay, and A. K. Singh. “A New Approach on Approximate Controllability of Sobolev-Type Hilfer Fractional Differential Equations”. An International Journal of Optimization and Control: Theories & Applications (IJOCTA), vol. 13, no. 1, Jan. 2023, pp. 130-8, doi:10.11121/ijocta.2023.1256.