Kronecker sequences beyond the torus: nearest-neighbour distances and best returns

Abstract

We study nearest-neighbour distances and strict best returns of cyclic orbits on $(V\times G)/\Lambda$, where $V$ is a finite-dimensional real normed space, $G$ has a bi-invariant ultrametric, and $\Lambda$ is an arbitrary closed subgroup. For the quotient of the maximum product metric, the number of distinct nearest-neighbour distances in every finite orbit segment is at most $P(\overline V)+1$, where $P$ is an open-ball packing constant and $\overline V$ is obtained by factoring out the continuous real part of $\Lambda$. For inner-product norms the bound depends only on the span of the projected subgroup. This gives $2^d+1$ for the maximum norm on $\mathbb R^d$ and strong-kissing-number bounds for inner-product norms, extending the corresponding real and adelic distance bounds. Our second main result is an exact abelianization principle: every such cyclic orbit is isometric, with its integer indexing preserved, to a cyclic orbit on an abelian mixed quotient $(V\times U)/\Lambda_{\mathrm{ab}}$. The construction preserves the reduced and effective real factors. Thus noncommutativity and non-normality enlarge the class of ambient presentations but not the class of cyclic-orbit metrics. The record-count identity and its growth consequences require only an isometric embedding of a metric space. For mixed homogeneous spaces we prove packing recurrences and, using abelianization, the sharp three-step and five-step recurrences in effective Euclidean dimensions one and two, without a normalizer assumption. We also extend the purely real recurrence to noncompact quotients and give counterexamples showing why the stronger real recurrence can fail after an ultrametric factor is added.