@article{CMH_1964__39_139280,
     author = {Joseph A. Wolf},
     title = {Isotropic {Manifolds} of {Indefinite} {Metric.}},
     journal = {Commentarii mathematici Helvetici},
     pages = {21--64},
     publisher = {mathdoc},
     volume = {39},
     year = {1964},
     zbl = {0125.39203},
     url = {http://geodesic.mathdoc.fr/item/CMH_1964__39_139280/}
}