Simultaneous Busemann-Petty and Shephard Volume Comparisons

Abstract

We study the simultaneous Busemann-Petty and Shephard volume comparison problem: whether comparison of the volumes of all central hyperplane sections and all orthogonal hyperplane projections determines the ordering of the volumes of two convex bodies. For every $n\ge 5$, we construct origin-symmetric convex bodies of revolution $K,L\subset {\mathbb{R}}^n$ such that every central hyperplane section and every orthogonal hyperplane projection of $K$ has strictly smaller volume than the corresponding section or projection of $L$, while $|K|>|L|$. For $n\le 4$, the affirmative solution of the Busemann--Petty problem shows that the section inequalities alone imply $|K|\le |L|$. Without origin symmetry, we construct such counterexamples in every dimension $n\ge 2$, with one body a nontrivial translate of a Euclidean ball and the other a noncentrally symmetric body of constant brightness.