On the diameter of separated point sets with many nearly equal distances

Authors:
János Pach;Radoš Radoičić;Jan Vondrák
Affiliations:
City College, CUNY and Courant Institute of Mathematical Sciences, New York University, New York, NY;Department of Mathematics, Rutgers University, New Brunswick, NJ;Departmemt of Mathematics, Massachttsetts Institute of Technology Cambridge, MA
Venue:
European Journal of Combinatorics - Special issue on extremal and probabilistic combinatorics
Year:
2006

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Abstract

A point set is separated if the minimum distance between its elements is 1. We call two real numbers nearly equal if they differ by at most 1. We prove that for any dimension d ≥ 2 and any γ 0, if P is a separated set of n points in Rd such that at least γn2 pairs in (P 2) determine nearly equal distances, then the diameter of P is at least C(d, γ)n2/(d-1) for some constant C(d, γ) 0. In the case of d = 3, this result confirms a conjecture of Erdös. The order of magnitude of the above bound cannot be improved for any d.