@article{SIGMA_2009_5_a65,
     author = {Graham S. Hall and David P. Lonie},
     title = {Holonomy and {Projective} {Equivalence} in {4-Dimensional} {Lorentz} {Manifolds}},
     journal = {Symmetry, integrability and geometry: methods and applications},
     year = {2009},
     volume = {5},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SIGMA_2009_5_a65/}
}