“…If we determine the set of self-adjoint extensions of system A, that we denote by M A , shouldn't the set of self-adjoint extensions of the composite system M AB be such that M AB = M A ? It is well known that in this case we have that M A = U(1) , see for instance [AIM05,ILPP13,IPP13,Koc75,BGP08]. However, as we will see by means of a computation later, the set of self-adjoint extensions of the composite system is much bigger.…”