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A212503
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Number of (w,x,y,z) with all terms in {1,...,n} and w<2x and y<2z.
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2
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0, 1, 9, 49, 144, 361, 729, 1369, 2304, 3721, 5625, 8281, 11664, 16129, 21609, 28561, 36864, 47089, 59049, 73441, 90000, 109561, 131769, 157609, 186624, 219961, 257049, 299209, 345744, 398161, 455625, 519841, 589824, 667489, 751689
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OFFSET
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0,3
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COMMENTS
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For a guide to related sequences, see A211795.
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LINKS
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FORMULA
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a(n) = 2*a(n-1)+2*a(n-2)-6*a(n-3)+6*a(n-5)-2*a(n-6)-2*a(n-7)+a(n-8).
G.f.: x*(1+x+x^2)*(1+6*x+22*x^2+6*x^3+x^4)/((1+x)^3*(1-x)^5). [Bruno Berselli, May 31 2012]
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MATHEMATICA
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t = Compile[{{n, _Integer}}, Module[{s = 0},
(Do[If[w < 2 x && y < 2 z, s = s + 1],
{w, 1, #}, {x, 1, #}, {y, 1, #}, {z, 1, #}] &[n]; s)]];
Map[t[#] &, Range[0, 40]] (* A212503 *)
LinearRecurrence[{2, 2, -6, 0, 6, -2, -2, 1}, {0, 1, 9, 49, 144, 361, 729, 1369}, 40] (* Harvey P. Dale, Jun 14 2017 *)
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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