@article{MOMA_1970__74_177494,
     author = {Erhard Heil},
     title = {Der {Vierscheitelsatz} in {Relativ-} und {Minkowski-Geometrie.}},
     journal = {Monatshefte f\"ur Mathematik},
     pages = {97--107},
     publisher = {mathdoc},
     volume = {74},
     year = {1970},
     zbl = {0175.48202},
     url = {http://geodesic.mathdoc.fr/item/MOMA_1970__74_177494/}
}