Parametric geometry of numbers over a number field and extension of scalars

Abstract

The parametric geometry of numbers of Schmidt and Summerer deals with rational approximation to points in ${\mathbb{R}}^n$. We extend this theory to a number field $K$ and its completion $K_w$ at a place $w$ in order to treat approximation over $K$ to points in $K_w^n$. As a consequence, we find that exponents of approximation over $\mathbb{Q}$ in ${\mathbb{R}}^n$ have the same spectrum as their generalizations over $K$ in $K_w^n$. When $w$ has relative degree one over a place $\ell$ of $\mathbb{Q}$, we further relate approximation over $K$ to a point $\mathbf{\xi }$ in $K_w^n$, to approximation over $\mathbb{Q}$ to a point $\Xi$ in ${\mathbb{Q}}_{\ell }^{nd}$, obtained by extension of scalars, where $d$ is the degree of $K$ over $\mathbb{Q}$. By combination with a result of Bel, this allows us to construct algebraic curves in ${\mathbb{R}}^{3d}$ defined over $\mathbb{Q}$, of degree $2d$, containing points that are very singular with respect to rational approximation.

40 pages, minor corrections, to appear in Bulletin de la Société Mathématique de France