Degree of mobility for metrics of lorentzian signature and parallel   (0,2)-tensor fields on cone manifolds
  Degree of mobility of a (pseudo-Riemannian) metric is the dimension of the space of metrics geodesically equivalent to it. We describe all possible values of the degree of mobility on a simply connected n-dimensional manifold of lorentz signature. As an application we calculate all possible differences between the dimension of the projective and the isometry groups. One of the main new technical results in the proof is the description of all parallel symmetric (0,2)-tensor fields on cone manifolds of signature $(n-1,2). 
Aleksandra Fedorova and Vladimir S. Matveev
math.DG math-ph math.AP math.MP