On beta-Plurality Points in Spatial Voting Games

Abstract

Let $V$ be a set of $n$ points in $\mathbb{R}^d$, called voters. A point $p\in \mathbb{R}^d$ is a plurality point for $V$ when the following holds: for every $q\in\mathbb{R}^d$ the number of voters closer to $p$ than to $q$ is at least the number of voters closer to $q$ than to $p$. Thus, in a vote where each $v\in V$ votes for the nearest proposal (and voters for which the proposals are at equal distance abstain), proposal $p$ will not lose against any alternative proposal $q$. For most voter sets a plurality point does not exist. We therefore introduce the concept of $β$-plurality points, which are defined similarly to regular plurality points except that the distance of each voter to $p$ (but not to $q$) is scaled by a factor $β$, for some constant $0<β\leq 1$. We investigate the existence and computation of $β$-plurality points, and obtain the following. * Define $β^*_d := \sup \{ β: \text{any finite multiset $V$ in $\mathbb{R}^d$ admits a $β$-plurality point} \}$. We prove that $β^*_2 = \sqrt{3}/2$, and that $1/\sqrt{d} \leq β^*_d \leq \sqrt{3}/2$ for all $d\geq 3$. * Define $β(p, V) := \sup \{ β: \text{$p$ is a $β$-plurality point for $V$}\}$. Given a voter set $V \in \mathbb{R}^2$, we provide an algorithm that runs in $O(n \log n)$ time and computes a point $p$ such that $β(p, V) \geq β^*_2$. Moreover, for $d\geq 2$ we can compute a point $p$ with $β(p,V) \geq 1/\sqrt{d}$ in $O(n)$ time. * Define $β(V) := \sup \{ β: \text{$V$ admits a $β$-plurality point}\}$. We present an algorithm that, given a voter set $V$ in $\mathbb{R}^d$, computes an $(1-\varepsilon)\cdot β(V)$ plurality point in time $O(\frac{n^2}{\varepsilon^{3d-2}} \cdot \log \frac{n}{\varepsilon^{d-1}} \cdot \log^2 \frac {1}{\varepsilon})$.