论文标题

重新审视当地跨越

Local Spanners Revisited

论文作者

Ashur, Stav, Har-Peled, Sariel

论文摘要

$ \ newCommand {\ emph} [1] {\ it {#1}} \ newCommand {\ ff} {\ Mathcal {f}} \ newCommand {\ newCommand {\ region} {\ sigry} {\ Mathsf {\ Mathsf {r}}}} \ newCommand {\ clasty {\ clasty {2]一组点$ p \ subseteq \ mathbb {r}^2 $和一个区域$ \ ff $,$ \ emph {local〜t-spanner} $ $ p $的{#2}} $,是$ p $的稀疏图$ g $,对于任何区域$ $ \ fff,用$ \限制{g} {\ region} = g_ {p \ cap \ region} $表示,对于$ \ region \ region \ cap p $的所有点来说,都是$ t $ -spanner。 我们介绍了用于建造本地跨度的算法,以相对于几个地区的家庭(例如凸区的同型)。不幸的是,所得图中的边数在对数取决于输入点集的扩展。我们证明无法删除这种依赖性,从而解决了Abam和Borouny提出的一个开放问题。我们还展示了对脂肪三角形和常规$ k $ gons的当地跨度的改进的构造(不依赖于差异)。特别是,这改善了已知的轴平行正方形结构。 我们还研究了当地扳手的较弱的概念,其中允许将区域缩小为“位”。如果收缩与直径成正比,则任何扳手都是弱的本地扳手。出乎意料的是,我们显示了轴平行矩形的弱扳手的接近线性构建,其中收缩为$ \ emph {乘法} $。

$\newcommand{\Emph}[1]{\it{#1}} \newcommand{\FF}{\mathcal{F}}\newcommand{\region}{\mathsf{r}}\newcommand{\restrictY}[2]{#1 \cap {#2}}$For a set of points $P \subseteq \mathbb{R}^2$, and a family of regions $\FF$, a $\Emph{local~t-spanner}$ of $P$, is a sparse graph $G$ over $P$, such that, for any region $\region \in \FF$, the subgraph restricted to $\region$, denoted by $\restrictY{G}{\region} = G_{P \cap \region}$, is a $t$-spanner for all the points of $\region \cap P$. We present algorithms for the construction of local spanners with respect to several families of regions, such as homothets of a convex region. Unfortunately, the number of edges in the resulting graph depends logarithmically on the spread of the input point set. We prove that this dependency can not be removed, thus settling an open problem raised by Abam and Borouny. We also show improved constructions (with no dependency on the spread) of local spanners for fat triangles, and regular $k$-gons. In particular, this improves over the known construction for axis parallel squares. We also study a somewhat weaker notion of local spanner where one allows to shrink the region a "bit". Any spanner is a weak local spanner if the shrinking is proportional to the diameter. Surprisingly, we show a near linear size construction of a weak spanner for axis-parallel rectangles, where the shrinkage is $\Emph{multiplicative}$.

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