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A note on the unit distance problem for planar configurations with Q-independent direction set

Metric Geometry 2014-06-26 v2 Combinatorics

Abstract

Let T(n)T(n) denote the maximum number of unit distances that a set of nn points in the Euclidean plane R2\mathbb{R}^2 can determine with the additional condition that the distinct unit length directions determined by the configuration must be Q\mathbb{Q}-independent. This is related to the Erdos unit distance problem but with a simplifying additional assumption on the direction set which holds "generically". We show that T(n+1)T(n)T(n+1)-T(n) is the Hamming weight of nn, i.e., the number of nonzero binary coefficients in the binary expansion of nn, and find a formula for T(n)T(n) explicitly. In particular T(n)T(n) is Θ(nlog(n))\Theta(n log(n)). Furthermore we describe a process to construct a set of nn points in the plane with Q\mathbb{Q}-independent unit length direction set which achieves exactly T(n)T(n) unit distances. In the process of doing this, we show T(n)T(n) is also the same as the maximum number of edges a subset of vertices of size nn determines in either the countably infinite lattice Z\mathbb{Z}^{\infty} or the infinite hypercube graph {0,1}\{0,1\}^{\infty}. The problem of determining T(n) can be viewed as either a type of packing or isoperimetric problem.

Keywords

Cite

@article{arxiv.1406.6029,
  title  = {A note on the unit distance problem for planar configurations with Q-independent direction set},
  author = {Mark Herman and Jonathan Pakianathan},
  journal= {arXiv preprint arXiv:1406.6029},
  year   = {2014}
}

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19 pages