Die theorie der geometrischen ordnungen

On a Theorem Related to the Four-Vertex Theorem

“…This conjecture has given rise to extensive investigations and a large literature (cf. surveys of Haupt [7] and Kiinneth [10]), but the proof has not been found. On the basis of ?4, as well as the investigation of similar constructions, the author has come to the opinion that such a proof is not possible.…”