2019 is coming! (2-soon ver.)

On a 2-dimensional plane, 2019 points are given such that, for every 3 points picked, there are 2 points whose distance is smaller than 1.

Is there a circle with radius 1 that can contain at least 1010 of the 2019 points?

Yes, of course No, there isn't

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1 solution

Zico Quintina
May 11, 2018

Suppose the statement is false. Take any one point P P and construct a unit circle C P C_P with center P P . By our assumption, C P C_P contains no more than 1009 points, so there are at least 1010 points outside of C P C_P .

Take any three points outside C P C_P , call them A , B A, B and C C . Then A P , B P AP, BP and C P CP are all greater than 1, so A B , B C AB, BC and A C AC must all be less than 1.

If we "push" A , B A, B and C C as far apart as possible under this constraint, they will be at the vertices of an equilateral triangle of side length 1 (technically just under 1 but that should not affect the proof.)

Now consider any other point X X also outside C P C_P . Since X P XP is greater than 1, X A , X B XA, XB and X C XC must all be less than 1, i.e X X must lie in the intersection of the unit circles centered at A , B A, B and C C (the shaded portion in the left image below). Since X X could be any point outside of C P C_P , this means that all points outside of C P C_P must lie in that intersection. But clearly that intersection can itself be contained within a unit circle, as shown in the right image.

So all points outside of C P C_P must lie within that unit circle, and there are at least 1010 such points.

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