Conformally flat homogeneous pseudo-Riemannian four-manifolds (Q2453150)

The authors describe the possible form of the Ricci operator at an arbitrary point of a four dimensional conformally flat pseudo-Riemannian manifold. They show that a fundamental distinction arises in terms of isotropy between the cases of non-degenerate and degenerate Ricci operators. Two different approaches are used when the isotropy is trivial. Finally, they classify all conformally flat pseudo-Riemannian homogeneous four-dimensional manifold with nontrivial isotropy and non-diagonalizable Ricci operator, which are not locally symmetric.