A simplicial polytope that maximizes the isotropic constant must be a simplex

TL;DR

This paper proves that among simplicial polytopes, the maximizer of the isotropic constant must be a simplex, advancing understanding of the geometric structures that optimize this measure.

Contribution

It shows that the maximizer of the isotropic constant among simplicial polytopes must be a simplex, narrowing the class of potential maximizers.

Findings

Maximizers are symmetric around hyperplanes spanned by faces and the origin.

Maximizers among simplicial polytopes are necessarily simplices.

Progress towards identifying the shape that maximizes the isotropic constant.

Abstract

The isotropic constant $L_K$ is an affine-invariant measure of the spread of a convex body $K$. For a $d$-dimensional convex body $K$, $L_K$ can be defined by $L_K^{2d} = \det(A(K))/(\mathrm{vol}(K))^2$, where $A(K)$ is the covariance matrix of the uniform distribution on $K$. It is an outstanding open problem to find a tight asymptotic upper bound of the isotropic constant as a function of the dimension. It has been conjectured that there is a universal constant upper bound. The conjecture is known to be true for several families of bodies, in particular, highly symmetric bodies such as bodies having an unconditional basis. It is also known that maximizers cannot be smooth. In this work we study the gap between smooth bodies and highly symmetric bodies by showing progress towards reducing to a highly symmetric case among non-smooth bodies. More precisely, we study the set of maximizers among simplicial polytopes and we show that if a simplicial polytope $K$ is a maximizer of the isotropic constant among $d$-dimensional convex bodies, then when $K$ is put in isotropic position it is symmetric around any hyperplane spanned by a $(d-2)$-dimensional face and the origin. By a result of Campi, Colesanti and Gronchi, this implies that a simplicial polytope that maximizes the isotropic constant must be a simplex.