Ellipses of minimal area and of minimal eccentricity circumscribed about a convex quadrilateral

TL;DR

This paper investigates the properties of ellipses circumscribed about convex quadrilaterals, proving the existence and uniqueness of minimal eccentricity and minimal area ellipses, and introduces the concept of bielliptic quadrilaterals.

Contribution

It establishes the existence and uniqueness of minimal eccentricity and minimal area circumscribed ellipses for convex quadrilaterals and introduces the concept of bielliptic quadrilaterals.

Findings

Unique ellipse of minimal eccentricity exists for any convex quadrilateral.

Unique ellipse of minimal area exists for any convex quadrilateral.

Existence of bielliptic quadrilaterals that are not bicentric.

Abstract

First, we fill in key gaps in Steiner's nice characterization of the most nearly circular ellipse which passes through the vertices of a convex quadrilateral, D. Steiner proved that there is only one pair of conjugate directions, M1 and M2, that belong to all ellipses of circumscription. Then he proves that if there is an ellipse, E, whose equal conjugate diameters possess the directional constants M1 and M2, then E must be an ellipse of circumscription which has minimal eccentricity. However, Steiner does not show the existence or uniqueness of such an ellipse. We prove that there is a unique ellipse of minimal eccentricity which passes through the vertices of D. We also show that there exists an ellipse which passes through the vertices of D and whose equal conjugate diameters possess the directional constants M1 and M2. We also show that there exists a unique ellipse of minimal area which passes through the vertices of D. Finally, we call a convex quadrilateral, D, bielliptic if the unique inscribed and circumscribed ellipses of minimal eccentricity have the same eccentricity. This generalizes the notion of bicentric quadrilaterals. In particular we show the existence of a bielliptic convex quadrilateral which is not bicentric.