On Lorentz manifolds with essential conformal group (Q1777252)

The author constructs compact Lorentzian manifolds \((M,g)\) the conformal group of which is essential, i.e. not the isometry group of a manifold conformal to \((M,g)\). He shows this for quotients of the Einstein universe \(S^1\times S^{n-1}\), the connected sum of \(g\) copies of \(S^1\times S^{n-2}\), and for \(S^1\times\Sigma_g\). Here \(S^n\) denotes the round \(n\)-sphere and \(\Sigma_g\) an oriented surface of genus \(g\). The Lorentzian case is in contrast with the Ferrand-Obata theorem for the Riemannian case.