Eg. Kalnins et Gc. Williams, INTEGRABILITY CONDITIONS FOR THE COUPLED ELECTROMAGNETIC-GRAVITATIONAL PERTURBATION EQUATIONS OF THE KERR-NEWMAN SPACETIME, Classical and quantum gravity, 15(5), 1998, pp. 1331-1338
We show by considering integrability conditions for the perturbation e
quations of the Kerr-Newman spacetime that it is possible, in principl
e, to obtain decoupled equations of eighth order for at least one of t
he perturbed quantities.