An equichordal characterization of the ellipsoid and the sphere

Abstract

Let $K$ and $L$ be two convex bodies in ${\mathbb{R}}^n$, $n\ge 3$, with $L\subset \text{int}K$. In this paper we prove the following result: if every two parallel chords of $K$, supporting $L$ have the same length, then $K$ and $L$ are homothetic and concentric ellipsoids. We also prove a similar theorem when instead of parallel chords we consider concurrent chords. We may also replace, in both theorems, supporting chords of $L$ by supporting sections of constant width. In the last section we also prove similar theorems where we consider projections instead of sections.