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A206687
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Number of n X 2 0..3 arrays with no element equal to another within two positions in the same row or column, and new values 0..3 introduced in row major order.
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3
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1, 4, 11, 36, 116, 376, 1216, 3936, 12736, 41216, 133376, 431616, 1396736, 4519936, 14626816, 47333376, 153174016, 495681536, 1604059136, 5190844416, 16797925376, 54359228416, 175910158336, 569257230336, 1842155094016, 5961339109376
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OFFSET
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1,2
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COMMENTS
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LINKS
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FORMULA
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Empirical: a(n) = 2*a(n-1) + 4*a(n-2) for n>4.
G.f.: x*(1 - x)*(1 + x)*(1 + 2*x) / (1 - 2*x - 4*x^2).
a(n) = ((1-sqrt(5))^n*(-5+3*sqrt(5)) + (1+sqrt(5))^n*(5+3*sqrt(5))) / (16*sqrt(5)) for n>2.
(End)
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EXAMPLE
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All solutions for n=4:
..0..1....0..1....0..1....0..1....0..1....0..1....0..1....0..1....0..1
..2..3....1..0....2..3....1..0....2..3....1..2....1..2....1..2....1..0
..1..2....2..3....1..0....2..3....3..2....2..0....3..0....2..3....2..3
..0..1....3..2....3..1....3..1....1..0....3..1....0..1....0..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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