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A208079
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Number of 4 X n 0..1 arrays avoiding 0 0 0 and 0 0 1 horizontally and 0 0 0 and 0 1 0 vertically.
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1
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9, 81, 441, 2601, 15129, 88209, 514089, 2996361, 17464041, 101787921, 593263449, 3457792809, 20153493369, 117463167441, 684625511241, 3990289900041, 23257113888969, 135552393433809, 790057246713849, 4604791086849321
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OFFSET
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1,1
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COMMENTS
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LINKS
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FORMULA
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Empirical: a(n) = 5*a(n-1) + 5*a(n-2) - a(n-3).
G.f.: 9*x*(1 + 4*x - x^2) / ((1 + x)*(1 - 6*x + x^2)).
a(n) = (9/4)*(2*(-1)^n + (3-2*sqrt(2))^n + (3+2*sqrt(2))^n).
(End)
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EXAMPLE
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Some solutions for n=4:
..1..0..1..0....1..1..0..1....1..1..1..1....0..1..1..0....1..1..1..0
..1..0..1..1....1..0..1..0....0..1..1..0....1..1..0..0....0..1..1..1
..0..1..0..1....1..0..1..1....0..1..1..1....1..0..1..1....1..1..0..1
..1..1..1..0....1..1..0..1....1..1..0..1....0..1..1..1....1..0..1..0
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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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