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Source: 2017 USAMO #5

April 20, 2017
AMCUSA(J)MOUSAMO2017 USAMOHi

Problem Statement

Let Z\mathbf{Z} denote the set of all integers. Find all real numbers c>0c > 0 such that there exists a labeling of the lattice points (x,y)∈Z2 ( x, y ) \in \mathbf{Z}^2 with positive integers for which:
[*] only finitely many distinct labels occur, and [*] for each label ii, the distance between any two points labeled ii is at least cic^i.
Proposed by Ricky Liu