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A221510
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Number of 0..3 arrays of length n with each element differing from at least one neighbor by 2 or more, starting with 0.
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1
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0, 2, 3, 12, 30, 89, 248, 706, 1995, 5652, 15998, 45297, 128240, 363074, 1027923, 2910236, 8239390, 23327177, 66043368, 186980482, 529374875, 1498754084, 4243238398, 12013359841, 34011950560, 96293859202, 272624979363
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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) = a(n-1) +4*a(n-2) +3*a(n-3) +a(n-4).
Empirical g.f.: x^2*(2 + x + x^2) / ((1 + x)*(1 - 2*x - 2*x^2 - x^3)). - Colin Barker, Oct 18 2017
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EXAMPLE
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Some solutions for n=6
..0....0....0....0....0....0....0....0....0....0....0....0....0....0....0....0
..2....2....2....2....2....3....2....2....3....3....2....3....3....2....3....3
..3....0....2....1....2....0....0....3....1....3....3....2....0....1....2....3
..0....2....0....3....0....3....1....1....0....1....1....0....3....3....0....1
..3....2....2....0....3....0....3....1....3....3....0....3....3....3....0....0
..0....0....0....3....1....2....1....3....0....0....2....0....1....1....2....3
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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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