Radii of Euclidean sections of $\ell_p$-balls

TL;DR

This paper investigates the radius of Euclidean sections of $\, ext{l}_p$-balls, especially the $ ext{l}_1$-ball, highlighting new bounds, survey of related results, and discussing algorithmic challenges relevant to theoretical computer science.

Contribution

It provides new insights into the radius of Euclidean sections of $ ext{l}_p$-balls, especially the cross-polytope, and surveys related results across various spaces with open problems and algorithmic considerations.

Findings

Improved bounds on the radius of Euclidean sections of $ ext{l}_1$-balls.

Survey of results for $ ext{l}_p$-spaces and non-commutative analogues.

Discussion of algorithmic challenges in finding nearly spherical sections.

Abstract

The celebrated Dvoretzky theorem asserts that every $N$-dimensional convex body admits central sections of dimension $d = \Omega(\log N)$, which is nearly spherical. For many instances of convex bodies, typically unit balls with respect to some norm, much better lower bounds on $d$ have been obtained, with most research focusing on such lower bounds and on the degree of approximation of the section by a $d$-dimensional Euclidean ball. In this note we concentrate on another parameter, namely the radius of the approximating ball. We focus on the case of the unit ball of the space $\ell_1^N$ (the so-called cross-polytope), which is relevant to various questions of interest in theoretical computer science. We will also survey other instances where similar questions for other normed spaces (most often $\ell_p$-spaces or their non-commutative analogues) were found relevant to problems in various areas of mathematics and its applications, and state some open problems. Finally, in view of the computer science ramifications, we will comment on the algorithmic aspects of finding nearly spherical sections.