The Hadwiger-Nelson problem over certain fields

Abstract

We compute the Hadwiger-Nelson numbers $\chi(E^2)$ for certain number fields $E$, that is, the smallest number of colors required to color the points in the plane with coordinates in~$E$ so that no two points at distance $1$ from one another have the same color. Specifically, we show that $\chi(\mathbb{Q}(\sqrt{2})^2) = 2$, that $\chi(\mathbb{Q}(\sqrt{3})^2) = 3$, that $\chi(\mathbb{Q}(\sqrt{7})^2) = 3$ despite the fact that the graph $\Gamma(\mathbb{Q}(\sqrt{7})^2)$ is triangle-free, and that $4 \leq \chi(\mathbb{Q}(\sqrt{3}, \sqrt{11})^2) \leq 5$. We also discuss some results over other fields, for other quadratic fields. We conclude with some comments on the use of the axiom of choice.