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A319159
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Given an equilateral triangular grid with side n, containing n(n+1)/2 points, a(n) is the minimal number of points to be selected, such that any equilateral triangle of points will include at least one of the selection.
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1
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1, 2, 4, 7, 11, 16, 22, 28, 35, 44, 53, 63, 74, 86
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OFFSET
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1,2
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COMMENTS
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This is the complementary problem to A240114: a(n) + A240114(n) = n(n+1)/2.
This is the same problem as A227116 and A319158, except that here the triangles may have any orientation. Due to the additional requirements, a(n) >= A227116(n) >= A319158(n).
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LINKS
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EXAMPLE
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For n=4, this sequence has the same value a(4)=4 as A227116 and A319158, but if we look at the three solutions to those sequences (unique up to symmetry), representing selected points by O:
O O O
O , . , . .
, . O , O . . O .
. O , . O . , O . O O .
We see that only the last of these is a solution here -- the others have rotated triangles not including any selected point (for example, as shown with commas). The last selection is therefore the unique solution (up to symmetry) for a(4)=4.
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CROSSREFS
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KEYWORD
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nonn,more,hard
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AUTHOR
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STATUS
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approved
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