W. Kuhnel et Hb. Rademacher, CONFORMAL DIFFEOMORPHISMS PRESERVING THE RICCI TENSOR, Proceedings of the American Mathematical Society, 123(9), 1995, pp. 2841-2848
We characterize semi-Riemannian manifolds admitting a global conformal
transformation such that the difference of the two Ricci tensors is a
constant multiple of the metric. Unless the conformal transformation
is homothetic, the only possibilities are standard Riemannian spaces o
f constant sectional curvature and a particular warped product with a
Ricci flat Riemannian manifold.