On the L\"owner-John Ellipsoids of the Metric Polytope

Abstract

The collection of all $n$-point metric spaces of diameter $\le 1$ constitutes a polytope $\mathcal{M}_n \subset \mathbb{R}^{\binom{n}{2}}$, called the \emph{Metric Polytope}. In this paper, we consider the best approximations of $\mathcal{M}_n$ by ellipsoids. We give an exact explicit description of the largest volume ellipsoid contained in $\mathcal{M}_n$. When inflated by a factor of $\Theta(n)$, this ellipsoid contains $\mathcal{M}_n$. It also turns out that the least volume ellipsoid containing $\mathcal{M}_n$ is a ball. When shrunk by a factor of $\Theta(n)$, the resulting ball is contained in $\mathcal{M}_n$. We note that the general theorems on such ellipsoid posit only that the pertinent inflation/shrinkage factors can be made as small as $O(n^2)$.