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A081268
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Diagonal of triangular spiral in A051682.
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4
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1, 12, 32, 61, 99, 146, 202, 267, 341, 424, 516, 617, 727, 846, 974, 1111, 1257, 1412, 1576, 1749, 1931, 2122, 2322, 2531, 2749, 2976, 3212, 3457, 3711, 3974, 4246, 4527, 4817, 5116, 5424, 5741, 6067, 6402, 6746, 7099, 7461, 7832, 8212, 8601, 8999, 9406
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OFFSET
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0,2
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LINKS
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FORMULA
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a(n) = C(n, 0) + 11C(n, 1) + 9C(n, 2); binomial transform of (1, 11, 9, 0, 0, 0, ...).
a(n) = (9n^2 + 13n+2)/2.
G.f.: (1 + 9x - x^2)/(1-x)^3.
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EXAMPLE
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a(1) = 9*1 + 1 + 2 = 12;
a(2) = 9*2 + 12 + 2 = 32;
a(3) = 9*3 + 32 + 2 = 61.
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PROG
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CROSSREFS
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KEYWORD
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easy,nonn
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AUTHOR
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STATUS
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approved
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