Johnson, Murugesan, et al. “An Analysis of Approximate Controllability for Hilfer Fractional Delay Differential Equations of Sobolev Type Without Uniqueness”. Nonlinear Analysis: Modelling and Control, vol. 28, no. 4, Apr. 2023, pp. 632-54, https://doi.org/10.15388/namc.2023.28.32118.