@article{GDZPPN002036975, title = {On the four-vertex theorem.}, journal = {Aequationes mathematicae}, volume = {33}, year = {1987}, pages = {221-230}, zbl = {0635.53003}, url = {http://dml.mathdoc.fr/item/GDZPPN002036975} }
Pinkall, U. On the four-vertex theorem.. Aequationes mathematicae, Tome 33 (1987) pp. 221-230. http://gdmltest.u-ga.fr/item/GDZPPN002036975/