Title: 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. Mačaj and the second author. The
proof is nonconstructive, using the ultraproduct construction and a slight
portion of nonstandard analysis.