@article{SF_1991__43_3_135120,
     author = {P. Ehrlich},
     title = {Null cones and pseudo-riemannian metrics.},
     journal = {Semigroup forum},
     pages = {337--343},
     publisher = {mathdoc},
     volume = {43},
     number = {3},
     year = {1991},
     zbl = {0735.53016},
     url = {http://geodesic.mathdoc.fr/item/SF_1991__43_3_135120/}
}