Light Euclidean Steiner Spanners in the Plane

TL;DR

This paper proves that in the plane, Steiner $(1+oldsymbol{ ext{epsilon}})$-spanners can be constructed with optimal lightness proportional to $oldsymbol{ ext{epsilon}}^{-1}$, closing the gap between known bounds for Euclidean spanners.

Contribution

The authors present a construction of Steiner $(1+oldsymbol{ ext{epsilon}})$-spanners in the plane with optimal lightness $O(oldsymbol{ ext{epsilon}}^{-1})$, matching the lower bound and improving previous results.

Findings

Achieved tight lightness bound $O( ext{epsilon}^{-1})$ in the plane.

Introduced generalized shallow light trees for spanner construction.

Utilized directional spanners and modified window partitioning for analysis.

Abstract

Lightness is a fundamental parameter for Euclidean spanners; it is the ratio of the spanner weight to the weight of the minimum spanning tree of a finite set of points in $\mathbb{R}^d$. In a recent breakthrough, Le and Solomon (2019) established the precise dependencies on $\varepsilon>0$ and $d\in \mathbb{N}$ of the minimum lightness of $(1+\varepsilon)$-spanners, and observed that additional Steiner points can substantially improve the lightness. Le and Solomon (2020) constructed Steiner $(1+\varepsilon)$-spanners of lightness $O(\varepsilon^{-1}\log\Delta)$ in the plane, where $\Delta\geq \Omega(\sqrt{n})$ is the \emph{spread} of the point set, defined as the ratio between the maximum and minimum distance between a pair of points. They also constructed spanners of lightness $\tilde{O}(\varepsilon^{-(d+1)/2})$ in dimensions $d\geq 3$. Recently, Bhore and T\'{o}th (2020) established a lower bound of $\Omega(\varepsilon^{-d/2})$ for the lightness of Steiner $(1+\varepsilon)$-spanners in $\mathbb{R}^d$, for $d\ge 2$. The central open problem in this area is to close the gap between the lower and upper bounds in all dimensions $d\geq 2$. In this work, we show that for every finite set of points in the plane and every $\varepsilon>0$, there exists a Euclidean Steiner $(1+\varepsilon)$-spanner of lightness $O(\varepsilon^{-1})$; this matches the lower bound for $d=2$. We generalize the notion of shallow light trees, which may be of independent interest, and use directional spanners and a modified window partitioning scheme to achieve a tight weight analysis.