Orthogonal separation of variables on manifolds with constant curvature (Q1207168)

It is proved that on a Riemannian manifold with constant curvature and on a Lorentzian manifold with constant positive curvature every system of separable coordinates has an orthogonal equivalent, i.e. that in these manifolds the integration by separation of variables of the geodesic Hamilton-Jacobi equation always occurs in orthogonal coordinates. Proofs of this property concerning strictly-Riemannian manifolds of positive, negative and zero constant curvature (and also for conformally flat manifolds) were first given by Kalnins and Miller (1982-1986). The proof presented here is based on elementary properties of Killing vectors of an affine space and on a geometrical characterization of the equivalence classes of separable coordinates.