Shephard's problem
# Shephard's problem

In mathematics, Shephard's problem, is the following geometrical question asked by Geoffrey Colin Shephard in 1964: if K and L are centrally symmetric convex bodies in n-dimensional Euclidean space such that whenever K and L are projected onto a hyperplane, the volume of the projection of K is smaller than the volume of the projection of L, then does it follow that the volume of K is smaller than that of L?

In this case, "centrally symmetric" means that the reflection of K in the origin, −K, is a translate of K, and similarly for L. If $π_k$ : $R^n$ → $Π_k$ is a projection of $R^n$ onto some k-dimensional hyperplane $Π_k$ (not necessarily a coordinate hyperplane) and $V_k$ denotes k-dimensional volume, Shephard's problem is to determine the truth or falsity of the implication

    $V_{k}(\pi _{k}(K))\leq V_{k}(\pi _{k}(L)){\mbox{ for all }}1\leq k<n\implies V_{n}(K)\leq V_{n}(L).$

$V_k(π_k(K))$ is sometimes known as the brightness of K and the function $V_k$ o $π_k$ as a (k-dimensional) brightness function.

In dimensions n = 1 and 2, the answer to Shephard's problem is "yes". In 1967, however, Petty and Schneider showed that the answer is "no" for every n ≥ 3. The solution of Shephard's problem requires Minkowski's first inequality for convex bodies and the notion of projection bodies of convex bodies.

