“…More precisely, they exploited the conservation and positivity of the Dirac current to show that, because of the matching conditions, the only way in which a bound state solution of the Dirac equation can be normalizable is that each term in the Fourier expansion of the spinor field in time, and the angular variable around the axis of symmetry, be identically zero. The case of the extreme Kerr metric has been treated by [6], where necessary and sufficient conditions for the existence of bound states have been derived. However, since such conditions are expressed in terms of a complicated set of equations and inequalities, the author did not further study the problem of whether or not there exist values of the particle energy satisfying the aforementioned conditions.…”