Spanners in Planar Domains via Steiner Spanners and non-Steiner Tree Covers
<|reference_start|>Spanners in Planar Domains via Steiner Spanners and non-Steiner Tree Covers: We study spanners in planar domains, including polygonal domains, polyhedral terrain, and planar metrics. Previous work showed that for any constant $\epsilon\in (0,1)$, one could construct a $(2+\epsilon)$-spanner with $O(n\log(n))$ edges (SICOMP 2019), and there is a lower bound of $\Omega(n^2)$ edges for any $(2-\epsilon)$-spanner (SoCG 2015). The main open question is whether a linear number of edges suffices and the stretch can be reduced to $2$. We resolve this problem by showing that for stretch $2$, one needs $\Omega(n\log n)$ edges, and for stretch $2+\epsilon$ for any fixed $\epsilon \in (0,1)$, $O(n)$ edges are sufficient. Our lower bound is the first super-linear lower bound for stretch $2$.   En route to achieve our result, we introduce the problem of constructing non-Steiner tree covers for metrics, which is a natural variant of the well-known Steiner point removal problem for trees (SODA 2001). Given a tree and a set of terminals in the tree, our goal is to construct a collection of a small number of dominating trees such that for every two points, at least one tree in the collection preserves their distance within a small stretch factor. Here, we identify an unexpected threshold phenomenon around $2$ where a sharp transition from $n$ trees to $\Theta(\log n)$ trees and then to $O(1)$ trees happens. Specifically, (i) for stretch $ 2-\epsilon$, one needs $\Omega(n)$ trees; (ii) for stretch $2$, $\Theta(\log n)$ tree is necessary and sufficient; and (iii) for stretch $2+\epsilon$, a constant number of trees suffice. Furthermore, our lower bound technique for the non-Steiner tree covers of stretch $2$ has further applications in proving lower bounds for two related constructions in tree metrics: reliable spanners and locality-sensitive orderings. Our lower bound for locality-sensitive orderings matches the best upper bound (STOC 2022).<|reference_end|>
@article{bhore2024spanners,
    title={Spanners in Planar Domains via Steiner Spanners and non-Steiner Tree
  Covers},
    author={Sujoy Bhore and Bal'azs Keszegh and Andrey Kupavskii and Hung Le and
  Alexandre Louvet and D"om"ot"or P'alv"olgyi and Csaba D. T'oth},
    journal={arXiv preprint arXiv:2404.05045},
    year={2024},
    archivePrefix={arXiv},
    eprint={2404.05045},
    primaryClass={cs.CG cs.DS}
}