Inequalities on Projected Volumes

Abstract

In this paper we study the following geometric problem: given $2^n-1$ real numbers $x_A$ indexed by the non-empty subsets $A\subset \{1,..,n\}$, is it possible to construct a body $T\subset \mathbb{R}^n$ such that $x_A=|T_A|$ where $|T_A|$ is the $|A|$-dimensional volume of the projection of $T$ onto the subspace spanned by the axes in $A$? As it is more convenient to take logarithms we denote by $\psi_n$ the set of all vectors $x$ for which there is a body $T$ such that $x_A=\log |T_A|$ for all $A$. Bollob\'as and Thomason showed that $\psi_n$ is contained in the polyhedral cone defined by the class of `uniform cover inequalities'. Tan and Zeng conjectured that the convex hull $\DeclareMathOperator{\conv}{conv}$ $\conv(\psi_n)$ is equal to the cone given by the uniform cover inequalities. We prove that this conjecture is `nearly' right: the closed convex hull $\overline{\conv}(\psi_n)$ is equal to the cone given by the uniform cover inequalities. However, perhaps surprisingly, we also show that $\conv (\psi_n)$ is not closed for $n\ge 4$, thus disproving the conjecture.