Quadrilaterals inscribed in convex curves

TL;DR

This paper classifies all quadrilaterals inscribed in convex Jordan curves, extending previous results by employing area arguments and analyzing singular points, and shows the equivalence of inscribing isosceles trapezoids in smooth and piecewise curves.

Contribution

It provides a complete classification of inscribed quadrilaterals in convex curves and introduces a simplified area-based proof approach, addressing a question posed by Makeev.

Findings

Classified all quadrilaterals inscribed in convex Jordan curves.

Established the equivalence of inscribing isosceles trapezoids in smooth and piecewise curves.

Extended the area argument method to this geometric problem.

Abstract

We classify the set of quadrilaterals that can be inscribed in convex Jordan curves, in the continuous as well as in the smooth case. This answers a question of Makeev in the special case of convex curves. The difficulty of this problem comes from the fact that standard topological arguments to prove the existence of solutions do not apply here due to the lack of sufficient symmetry. Instead, the proof makes use of an area argument of Karasev and Tao, which we furthermore simplify and elaborate on. The continuous case requires an additional analysis of the singular points, and a small miracle, which then extends to show that the problems of inscribing isosceles trapezoids in smooth curves and in piecewise $C^1$ curves are equivalent.