A uniform stability principle for dual lattices

Abstract: We prove a highly uniform stability or "almost-near" theorem for dual lattices of lattices $L \subseteq \Bbb R^n$. More precisely, we show that, for a vector $x$ from the linear span of a lattice $L \subseteq \Bbb R^n$, subject to $\lambda_1(L) \ge \lambda > 0$, to be $\varepsilon$-close to some vector from the dual lattice $L'$ of $L$, it is enough that the inner products $u\,x$ are $\delta$-close (with $\delta < 1/3$) to some integers for all vectors $u \in L$ satisfying $| u | \le r$, where $r > 0$ depends on $n$, $\lambda$, $\delta$ and $\varepsilon$, only. This generalizes an earlier analogous result proved for integral vector lattices by M. Ma\v{c}aj and the second author. The proof is nonconstructive, using the ultraproduct construction and a slight portion of nonstandard analysis.