Symmetries of Lorentzian Three-Manifolds with Recurrent Curvature
  Locally homogeneous Lorentzian three-manifolds with recurrect curvature are special examples of Walker manifolds, that is, they admit a parallel null vector field. We obtain a full classification of the symmetries of these spaces, with particular regard to symmetries related to their curvature: Ricci and matter collineations, curvature and Weyl collineations. Several results are given for the broader class of three-dimensional Walker manifolds. 
Giovanni Calvaruso, Amirhesam Zaeim
math.DG