Strange shadows of $\ell_p$-balls

TL;DR

This paper establishes a large deviations principle for projections and sections of $oldsymbol{ extit{ ext{l}}}_p$-balls, revealing their asymptotic geometric behavior and deriving new inequalities for the Gamma function.

Contribution

It introduces a large deviations framework for $ ext{l}_p$-ball projections and sections, characterizing their limit distributions and rate functions in high dimensions.

Findings

Projections tend to a Euclidean ball almost surely.

Identifies the rate function on $L_q$-zonoids and their duals.

Provides a new inequality for the Gamma function.

Abstract

We prove a large deviations principle for orthogonal projections of the unit ball $\mathbb{B}_p^n$ of $\ell_p^n$ onto a random $k$-dimensional linear subspace of $\mathbb{R}^n$ as $n\to\infty$ in the case $2<p\le \infty$ and for the intersection of $\mathbb{B}_p^n$ with a random $k$-dimensional subspace in the case $1\le p <2$. The corresponding rate function is finite only on $L_q$-zonoids and their duals, respectively, and given in terms of the maximum entropy over suitable measures generating the $L_q$-zonoid, where $\frac{1}{p}+\frac{1}{q}=1$. In particular, we obtain that the renormalized projections/sections almost surely tend to a $k$-dimensional Euclidean ball of certain radius. Moreover, we identify the asymptotic probability that the random orthogonal projection remains within a ball of smaller radius. As a byproduct we obtain an interesting inequality for the Gamma function.