@article{MZ_1988__198_3_173944,
     author = {Demir N. Kupeli},
     title = {On {Conjugate} and {Focal} {Points} in {Semi-Riemannian} {Geometry.}},
     journal = {Mathematische Zeitschrift},
     pages = {569--590},
     publisher = {mathdoc},
     volume = {198},
     number = {3},
     year = {1988},
     zbl = {0658.53059},
     url = {http://geodesic.mathdoc.fr/item/MZ_1988__198_3_173944/}
}