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A232921
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Number of 2 X n 0..2 arrays with no element x(i,j) adjacent to value 2-x(i,j) horizontally, diagonally or antidiagonally.
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1
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9, 18, 54, 144, 396, 1080, 2952, 8064, 22032, 60192, 164448, 449280, 1227456, 3353472, 9161856, 25030656, 68385024, 186831360, 510432768, 1394528256, 3809922048, 10408900608, 28437645312, 77693091840, 212261474304
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OFFSET
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1,1
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LINKS
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FORMULA
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Empirical: a(n) = 2*a(n-1) + 2*a(n-2).
G.f.: 9*x / (1 - 2*x - 2*x^2).
a(n) = (-3)*(sqrt(3)*((1-sqrt(3))^n - (1+sqrt(3))^n)) / 2.
(End)
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EXAMPLE
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Some solutions for n=5:
..0..1..2..2..2....2..1..2..2..1....2..1..0..0..1....2..2..1..0..1
..2..1..2..1..2....0..1..2..2..1....2..1..0..0..0....2..2..1..0..1
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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