Fubini Theorem for pseudo-Riemannian metrics
  We generalize the following classical result of Fubini for pseudo-Riemannian metrics: if three essentially different metrics on $M^{n\ge 3}$ share the same unparametrized geodesics, and two of them (say, $g$ and $\bar g$) are strictly nonproportional (i.e., the minimal polynomial of $g^{i\alpha} \bar g_{\alpha j}$ coincides with the characteristic polynomial) at least at one point, then they have constant curvature. 
Alexey V. Bolsinov, Volodymyr Kiosak, Vladimir S. Matveev