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A221537
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Number of 0..3 arrays of length n with each element differing from at least one neighbor by something other than 1, starting with 0.
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1
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0, 3, 8, 30, 103, 364, 1276, 4483, 15740, 55274, 194095, 681576, 2393384, 8404483, 29512736, 103635366, 363920471, 1277923892, 4487489988, 15758032643, 55335074484, 194311722642, 682334774239, 2396050726160, 8413845078800
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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) = 3*a(n-1) +2*a(n-2) -a(n-3) +a(n-4).
Empirical g.f.: x^2*(3 - x) / ((1 + x)*(1 - 4*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
..3....0....3....2....0....0....2....0....0....0....0....2....0....2....0....3
..3....0....1....2....0....0....2....2....3....3....1....2....3....2....0....3
..0....3....3....2....2....3....0....2....3....0....3....0....3....0....0....0
..1....1....3....0....1....3....0....2....3....0....3....1....1....1....0....0
..1....1....0....3....1....1....3....0....3....3....0....3....1....1....0....2
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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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