“…For the regional controllability problem, Ge et al studied the regional controllability of a linear diffusion system with a Caputo derivative of order α ∈ (0, 1] in two cases: when the control operator is bounded, by using the Hilbert Uniqueness Method (HUM) approach; and when one has an unbounded operator, by using a compactness condition [25]. Tajani et al established sufficient conditions for regional controllability of Riemann-Liouville fractional semilinear sub-diffusion systems using two approaches: an analytical approach, based on the contraction mapping theorem; and using the HUM approach, where the regional approximate controllability of the associated linear system is assumed to hold [26,27].…”