Sextactic points on a simple closed curve
Gudlaugur Thorbergsson, Masaaki Umehara · Nagoya Mathematical Journal · 2002
Abstract We give optimal lower bounds for the number of sextactic points on a simple closed curve in the real projective plane. Sextactic points are after inflection points the simplest projectively invariant singularities on such curves. Our method is axiomatic and can be applied in other situations.