@article{MAN_1978__232_163087,
     author = {Katsumi Nomizu and Larry Graves},
     title = {On {Sectional} {Curvature} of {Indefinite} {Metrics.}},
     journal = {Mathematische Annalen},
     pages = {267--272},
     publisher = {mathdoc},
     volume = {232},
     year = {1978},
     zbl = {0366.53007},
     url = {http://geodesic.mathdoc.fr/item/MAN_1978__232_163087/}
}