On an equichordal property of a pair of convex bodies

Abstract

Let $d\ge 2$ and let $K$ and $L$ be two convex bodies in ${\mathbb R^d}$ such that $L\subset \textrm{int}\,K$ and the boundary of $L$ does not contain a segment. If $K$ and $L$ satisfy the $(d+1)$-equichordal property, i.e., for any line $l$ supporting the boundary of $L$ and the points $\{\zeta_{\pm}\}$ of the intersection of the boundary of $K$ with $l$, $$ \textrm{dist}^{d+1}(L\cap l, \zeta_+)+\textrm{dist}^{d+1}(L\cap l, \zeta_-)=2\sigma^{d+1} $$ holds, where the constant $\sigma$ is independent of $l$, does it follow that $K$ and $L$ are concentric Euclidean balls? We prove that if $K$ and $L$ have $C^2$-smooth boundaries and $L$ is a body of revolution, then $K$ and $L$ are concentric Euclidean balls.