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A229447
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Number of 4 X n 0..2 arrays with horizontal differences mod 3 never 1, vertical differences mod 3 never -1, and rows and columns lexicographically nondecreasing.
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1
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12, 25, 53, 109, 212, 387, 665, 1083, 1684, 2517, 3637, 5105, 6988, 9359, 12297, 15887, 20220, 25393, 31509, 38677, 47012, 56635, 67673, 80259, 94532, 110637, 128725, 148953, 171484, 196487, 224137, 254615, 288108, 324809, 364917, 408637, 456180
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OFFSET
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1,1
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LINKS
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FORMULA
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Empirical: a(n) = (1/4)*n^4 - (1/3)*n^3 + (13/4)*n^2 + (11/6)*n + 7.
G.f.: x*(12 - 35*x + 48*x^2 - 26*x^3 + 7*x^4) / (1 - x)^5.
a(n) = 5*a(n-1) - 10*a(n-2) + 10*a(n-3) - 5*a(n-4) + a(n-5) for n>5.
(End)
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EXAMPLE
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Some solutions for n=4:
..0..0..0..2....0..0..0..2....0..0..0..2....0..2..2..2....0..0..0..0
..1..1..1..0....1..1..1..0....1..1..1..0....1..0..2..2....1..1..1..1
..2..2..2..1....1..1..1..0....1..1..1..1....1..0..2..2....2..2..2..2
..2..2..2..2....2..2..2..1....2..2..2..1....2..1..0..2....2..2..2..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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