The sharp exponent for the Banach--Mazur distance to the cube

Abstract

Let $R_\infty(n)$ be the maximal Banach--Mazur distance from an $n$-dimensional real normed space to $\ell_\infty^n$. We prove that $R_\infty(n)\ge cn^{2/3}(\log(2n))^{-10}$. Together with the upper bound of Friedland, Naor, and Youssef, this gives $R_\infty(n) = n^{2/3+o(1)}$. The examples are polars of symmetric Gaussian polytopes with $2n$ generators, and the lower bound holds with exponentially high probability. A joint minimization over coefficient representations and positive weights yields an exact second-variation identity and sparse coefficients. A Gaussian projection argument controls all low-rank completions of the recorded matrix. An adaptive choice of exceptional generators then reduces the remaining case to a fixed-shift Gaussian small-ball estimate over two finite nets.