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@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/} }
U. Pinkall. On the four-vertex theorem.. Aequationes mathematicae, Tome 34 (1987), pp. 221-230. http://geodesic.mathdoc.fr/item/AM2_1987__34_137241/