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A227116

Given an equilateral triangular grid with side n, containing n(n+1)/2 points, a(n) is the minimal number of points to be removed from the grid, so that, if 3 of the remaining points are chosen, they do not form an equilateral triangle with sides parallel to the grid.

0, 1, 2, 4, 7, 9, 14, 18, 23, 29, 36, 44, 52, 61, 71

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OFFSET

1,3

COMMENTS

This is the complementary problem to A227308.

Numbers found by an exhaustive computational search for all solutions (see history).

LINKS

Table of n, a(n) for n=1..15.

Heinrich Ludwig, Illustration of a(2)..a(15)

Ed Wynn, A comparison of encodings for cardinality constraints in a SAT solver, arXiv:1810.12975 [cs.LO], 2018.

FORMULA