Characterizations of the Kerr metric

For stationary, asymptotically flat solutions of Einstein's equations, covariant functionals of the metric variables are defined which characterize the Kerr metric uniquely. For instance, we obtain a generalization of the “Bach tensor” to stationary metrics, which vanishes if and only if the solution is Kerr. We also give a new interpretation of the “Schwarzschild-to-Kerr-transformation.” Our results might be applicable to simplify the proof of the uniqueness theorem for stationary black holes.