Proof of projective Lichnerowicz conjecture for pseudo-Riemannian   metrics with degree of mobility greater than two
  We prove an important partial case of the pseudo-Riemannian version of the projective Lichnerowicz conjecture stating that a complete manifold admitting an essential group of projective transformations is the round sphere (up to a finite cover). 
Volodymyr Kiosak, Vladimir S. Matveev