“…Now, thanks to the analyticity of (T (t)) t≥0 , the operator (−A) γ |D(A) is a finite-time L 2 -admissible observation for (T (t)) t≥0 , and that A has the finite-time L 2 -maximal regularity, which implies in turns that that ((T (t)) t≥0 , B, (−A) γ ) generates a L 2 -regular system (see [1]). Furthermore, the L 2 -regularity of ((T (t) t≥0 ), B,C) has been recently established in [4]). Consequently, the conditions of Theorem 2 are fulfilled and this ends the proof.…”