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Aug 27, 2019 at 10:45 comment added Joseph O'Rourke Seems correct.${}$
Aug 27, 2019 at 3:14 comment added Haoran Chen @JosephO'Rourke If $r<(2\sqrt{(n-1)^2+n^2})^{-1}\approx (2n)^{-1}$, no line can cross ($2$ or more intersections) three collinear dots on $n\times n$ grids. Then there could be $0.75r^{-1}$ dots based on Hall et al. result?
Aug 26, 2019 at 14:41 comment added Joseph O'Rourke Related: For exact no-three-in-a-line of points on a $n \times n$ grid, the best lower bound is $n(1.5 -o(n))$ and best upper bound is $2n$. Discussed by David Eppstein in Forbidden Configurations in Discrete Geometry.
Aug 26, 2019 at 2:31 answer added user44143 timeline score: 4
Aug 26, 2019 at 1:30 answer added Nate timeline score: 2
Aug 26, 2019 at 0:53 history edited user44143
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Aug 25, 2019 at 15:19 history edited LSpice CC BY-SA 4.0
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Aug 25, 2019 at 9:46 history asked Haoran Chen CC BY-SA 4.0