English

Many antipodes implies many neighbors

Combinatorics 2025-03-26 v1 Metric Geometry

Abstract

Suppose {x1,,xn}R2\left\{x_1, \dots, x_n\right\} \subset \mathbb{R}^2 is a set of nn points in the plane with diameter 1\leq 1, meaning xixj1\|x_i - x_j\| \leq 1 for all 1i,jn1 \leq i,j \leq n. We show that if there are many `antipodes', these are pairs of points of with distance 1ε\geq 1-\varepsilon, then there are many neighbors, these are pairs of points that are distance ε\leq \varepsilon. More precisely, we prove that for some universal c>0c>0, #{(i,j):xixjε}cε3/4(logε1)1/4#{(i,j):xixj1ε}. \# \left\{(i,j): \|x_i - x_j\| \leq \varepsilon\right\} \geq \frac{c \cdot \varepsilon^{3/4}}{\left( \log \varepsilon^{-1} \right)^{1/4}}\cdot \# \left\{(i,j): \|x_i - x_j\| \geq 1- \varepsilon\right\}. The inequality is very easy too prove with factor ε2\varepsilon^2 and easy with ε\varepsilon. The optimal rate might be ε1/2\varepsilon^{1/2} which is attained by several examples.

Keywords

Cite

@article{arxiv.2503.19792,
  title  = {Many antipodes implies many neighbors},
  author = {Stefan Steinerberger},
  journal= {arXiv preprint arXiv:2503.19792},
  year   = {2025}
}
R2 v1 2026-06-28T22:34:02.414Z