# Lorentzian structure

MilesReid
It is fairly easy to prove that each manifold can be given a Riemannian structure. The argument is standard: locally you give the riemannian structure and then you use partions of unity. This proof breaks down for signed metrics. Even for a manifold requiring only two charts. For example, I've been told that you cannot put on S^2 a metric with signature +1, -1. This is quite remarkable, since on S^2-{p} diffeo to R^2 you can! Any ideas on proving it?