@article{BAG_1988__27_138413,
     author = {H. Pottmann and O. R\"oschel},
     title = {\"Offnungsinvarianten der {Bahnregelfl\"achen} euklidischer {Zwangl\"aufe}},
     journal = {Beitr\"age zur Algebra und Geometrie},
     pages = {97--112},
     publisher = {mathdoc},
     volume = {27},
     year = {1988},
     url = {http://geodesic.mathdoc.fr/item/BAG_1988__27_138413/}
}