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Separate differential and integral transformations are introduced for the individual radial and angular equations governing the (infinitesimally) gauge invariant Newman–Penrose quantities which represent massless perturbations of the Kerr black hole. Using these new transformations it is shown, without need for numerical investigation or reference to the analytic behavior of the separation constant, that no unstable mode perturbations exist for any physical value of the spin of massless fields on the rotating black hole background.
B. F. Whiting (Thu,) studied this question.