Applications of Siegel's Lemma to a system of linear forms and its minimal points

Abstract

Consider a real matrix $Θ$ consisting of rows $(θ_{i,1},\ldots,θ_{i,n})$, for $1\leq i\leq m$. The problem of making the system linear forms $x_{1}θ_{i,1}+\cdots+x_{n}θ_{i,n}-y_{i}$ for integers $x_{j},y_{i}$ small naturally induces an ordinary and a uniform exponent of approximation, denoted by $w(Θ)$ and $\widehat{w}(Θ)$ respectively. For $m=1$, a sharp lower bound for the ratio $w(Θ)/\widehat{w}(Θ)$ was recently established by Marnat and Moshchevitin. We give a short, new proof of this result upon a hypothesis on the best approximation integer vectors associated to $Θ$. Our conditional result extends to general $m>1$ (but may not be optimal in this case). Moreover, our hypothesis is always satisfied in particular for $m=1, n=2$ and thereby unconditionally confirms a previous observation of Jarník. We formulate our results in the more general context of approximation of subspaces of Euclidean spaces by lattices. We further establish criteria upon which a given number $\ell$ of consecutive best approximation vectors are linearly independent. Our method is based on Siegel's Lemma.