“…Further, if system is null controllable in time T , and we stop controlling the system at time T , then for all t ⩾ T , it holds that y ( t )= y t ( t )=0 in Ω. However, the null controllability does not hold for some degenerate wave equations (e.g., a ( x )= x s , s ⩾2; [, theorem 1.2] or [, example 3.7,3.8]), that is, we cannot find any control such that the corresponding state of the degenerate wave equation equal to zero in the whole of space domain. Therefore, persistent regional null controllability is introduced.…”