Max-norm Ramsey Theory

TL;DR

This paper refines the understanding of how the minimum number of colours needed to avoid monochromatic isometric copies of certain metric spaces in max-norm spaces grows exponentially with dimension, providing exact values for some cases.

Contribution

It determines the exact exponential growth rate of the chromatic number for one-dimensional and some product metric spaces, and explores infinite metric spaces.

Findings

Exact growth rate for one-dimensional $ ext{M}$

Chromatic number tends to infinity for some infinite $ ext{M}$

Refined exponential bounds for specific metric spaces

Abstract

Given a metric space $\mathcal{M}$ that contains at least two points, the chromatic number $\chi\left(\mathbb{R}^n_{\infty}, \mathcal{M} \right)$ is defined as the minimum number of colours needed to colour all points of an $n$-dimensional space $\mathbb{R}^n_{\infty}$ with the max-norm such that no isometric copy of $\mathcal{M}$ is monochromatic. The last two authors have recently shown that the value $\chi\left(\mathbb{R}^n_{\infty}, \mathcal{M} \right)$ grows exponentially for all finite $\mathcal{M}$. In the present paper we refine this result by giving the exact value $\chi_{\mathcal{M}}$ such that $\chi\left(\mathbb{R}^n_{\infty}, \mathcal{M} \right) = (\chi_{\mathcal{M}}+o(1))^n$ for all 'one-dimensional' $\mathcal{M}$ and for some of their Cartesian products. We also study this question for infinite $\mathcal{M}$. In particular, we construct an infinite $\mathcal{M}$ such that the chromatic number $\chi\left(\mathbb{R}^n_{\infty}, \mathcal{M} \right)$ tends to infinity as $n \rightarrow \infty$.