R. Pandey, C. Shukla, A. Shukla, A. Upadhyay, and A. K. Singh, “A new approach on approximate controllability of Sobolev-type Hilfer fractional differential equations”, Int. J. Optim. Control, Theor. Appl. (IJOCTA), vol. 13, no. 1, pp. 130–138, Jan. 2023.