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A201081
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Number of -1..1 arrays of n elements with first and second differences also in -1..1.
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1
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3, 7, 13, 25, 47, 89, 169, 321, 609, 1155, 2191, 4157, 7887, 14963, 28387, 53855, 102173, 193841, 367751, 697689, 1323641, 2511185, 4764169, 9038483, 17147623, 32532117, 61719263, 117092515, 222145507, 421449879, 799566029, 1516920201
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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) = 2*a(n-1) -a(n-2) +a(n-3) +a(n-4).
Empirical g.f.: x*(3 + 2*x + x^2)*(1 + 3*x^2 - 2*x^3 + x^4) / ((1 - x)^3*(1 + x + x^2)^2). - Colin Barker, Feb 15 2018
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EXAMPLE
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Some solutions for n=10:
..1....1....1...-1....1....0....0...-1....1....1....1....1....0....0....0...-1
..1....0....0...-1....0....0...-1...-1....0....0....1....0...-1....1....1...-1
..0...-1....0....0....0...-1...-1....0...-1...-1....1...-1...-1....1....1....0
.-1...-1....0....0....1...-1....0....1...-1...-1....0...-1....0....1....1....0
.-1...-1....0....1....1....0....0....1....0...-1....0....0....1....0....1....0
.-1...-1...-1....1....0....0...-1....0....0...-1...-1....0....1...-1....1...-1
..0...-1...-1....1....0....0...-1...-1....0...-1...-1....0....0...-1....1...-1
..0....0...-1....0....0....1...-1...-1....0...-1....0....0....0...-1....1...-1
..0....0....0....0....1....1....0...-1...-1....0....0....0....0...-1....0....0
..1....0....1...-1....1....1....1...-1...-1....1...-1....1...-1...-1....0....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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