@article{AM2_1987__34_137241,
     author = {U. Pinkall},
     title = {On the four-vertex theorem.},
     journal = {Aequationes mathematicae},
     pages = {221--230},
     publisher = {mathdoc},
     volume = {34},
     year = {1987},
     zbl = {0635.53003},
     url = {http://geodesic.mathdoc.fr/item/AM2_1987__34_137241/}
}