On characteristic properties of the ellipsoid in terms of circumscribed cones of a convex body

Abstract

We strongly believe that in order to prove two important geometrical pro\-blems in convexity, namely, the G. Bianchi and P. Gruber's Conjecture \cite{bigru} and the J. A. Barker and D. G. Larman's Conjecture \cite{Barker}, it is necessary obtain new characteristic properties of the ellipsoid, which involves the notions defined in such problems. In this work we present a series of results which intent to be a progress in such direction: Let $L,K\subset \mathbb{R}^n$ be convex bodies, $n\geq 3$, and $L$ be a subset in the interior of $K$. Then each of the following conditions i), ii) and iii) implies that $L$ is an ellipsoid. i) $L$ is $O$-symmetric and, for every $x$ in the boundary of $K$, the support cone $S(L,x)$ is ellipsoidal. ii) there exists a point $p\in \mathbb{R}^n$ such that for every $x$ in the boundary of $K$, there exists a point $y$ in the boundary of $K$ and hyperplane $\Pi$, passing through $p$, such that \[ S(L,x)\cap S(L,y)=\Pi \cap \textrm{bd } K. \] iii) $K$ and $L$ are $O$-symmetric, every $x$ in the boundary of $K$ is a pole of $L$ and $\Omega_x:=S(L,x)\cap S(L,-x)$ is contained in the interior of $K$. In the case ii), $K$ is also an ellipsoid and it is concentric with $L$. On the other hand, let $K\subset \mathbb{R}^n$ be a $O$-symmetric convex body, $n\geq 3$, and let $B$ in $\mathbb{R}^n$ be a ball with centre at $O$. We are going to prove that if $B$ is small enough and all the sections of $K$ given by planes tangent to $B$ are $(n-1)$-ellipsoids, then $K$ is an $n$-ellipsoid.