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A221440
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Hilltop maps: number of n X 2 binary arrays indicating the locations of corresponding elements not exceeded by any horizontal or antidiagonal neighbor in a random 0..1 n X 2 array.
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1
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3, 9, 31, 105, 355, 1201, 4063, 13745, 46499, 157305, 532159, 1800281, 6090307, 20603361, 69700671, 235795681, 797691075, 2698569577, 9129195487, 30883847113, 104479306403, 353450961809, 1195716038943, 4045078385041, 13684402155875
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OFFSET
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1,1
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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) + a(n-2) + a(n-3).
Empirical g.f.: x*(3 + x^2) / (1 - 3*x - x^2 - x^3). - Colin Barker, Aug 05 2018
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EXAMPLE
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Some solutions for n=3:
..0..1....1..0....0..1....1..1....1..0....1..1....1..1....1..0....1..1....0..1
..1..1....0..1....1..1....1..1....1..1....0..1....1..0....1..0....1..1....0..1
..0..1....1..1....1..0....1..0....0..1....0..1....1..0....1..1....0..1....1..1
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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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