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A245158
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Number of length n 0..3 arrays with new values introduced in order from both ends.
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1
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1, 1, 2, 4, 9, 23, 65, 199, 653, 2275, 8313, 31439, 121637, 477307, 1888721, 7509799, 29940861, 119550419, 477742889, 1909988479, 7637856725, 30546970411, 122178444417, 488693854679, 1954733475629, 7818845822083, 31275198738905
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OFFSET
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1,3
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LINKS
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FORMULA
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Empirical: a(n) = 10*a(n-1) - 37*a(n-2) + 64*a(n-3) - 52*a(n-4) + 16*a(n-5) for n>6.
G.f.: x*(1 - 9*x + 29*x^2 - 43*x^3 + 31*x^4 - 11*x^5) / ((1 - x)^2*(1 - 2*x)^2*(1 - 4*x)).
a(n) = (-64 - 27*2^(2+n) + 4^n + 12*(32+3*2^n)*n) / 576 for n>1.
(End)
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EXAMPLE
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Some solutions for n=7:
..0....0....0....0....0....0....0....0....0....0....0....0....0....0....0....0
..0....0....1....0....1....1....1....1....1....1....1....0....0....1....1....1
..0....0....0....1....1....2....0....2....2....0....0....1....0....0....1....2
..0....1....0....1....0....0....1....2....1....2....2....0....0....0....2....1
..1....2....0....2....1....2....1....1....0....1....1....1....0....0....0....1
..0....1....0....1....0....1....0....0....0....1....0....0....0....1....1....0
..0....0....0....0....0....0....0....0....0....0....0....0....0....0....0....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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