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A221420
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Number of 2 X n arrays of occupancy after each element stays put or moves to some horizontal or antidiagonal neighbor, with every occupancy equal to zero or two.
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1
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0, 4, 10, 29, 84, 242, 696, 2000, 5744, 16492, 47344, 135900, 390080, 1119636, 3213616, 9223756, 26474016, 75985524, 218092816, 625967180, 1796642240, 5156696372, 14800673136, 42480669068, 121927372384, 349954089588
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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) = 4*a(n-1) - 3*a(n-2) - 2*a(n-4) for n>6.
Empirical g.f.: x^2*(4 - 6*x + x^2 - 2*x^3 + x^4) / (1 - 4*x + 3*x^2 + 2*x^4). - Colin Barker, Aug 05 2018
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EXAMPLE
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Some solutions for n=3:
..0..2..0....0..2..0....0..2..2....2..0..2....0..2..2....0..2..0....2..0..0
..2..0..2....2..2..0....0..2..0....0..2..0....0..0..2....0..2..2....2..2..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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