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A084367
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a(n) = n*(2*n+1)^2.
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2
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0, 9, 50, 147, 324, 605, 1014, 1575, 2312, 3249, 4410, 5819, 7500, 9477, 11774, 14415, 17424, 20825, 24642, 28899, 33620, 38829, 44550, 50807, 57624, 65025, 73034, 81675, 90972, 100949, 111630, 123039, 135200, 148137, 161874
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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) = n*( n*(2*n+1)+1 + n*(2*n+1)+2 + ... + n*(2*n+1)+2*n ).
Sum_{n>=1} 1/a(n) = 4 - 2*log(2) - Pi^2/4. - Amiram Eldar, Jul 21 2020
Sum_{n>=1} (-1)^(n+1)/a(n) = Pi/2 + log(2) + 2*G - 4, where G is Catalan's constant (A006752). - Amiram Eldar, Feb 08 2022
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EXAMPLE
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a(3) = 147 since 147 = 3*7^2.
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MATHEMATICA
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CoefficientList[Series[x*(9+14*x+x^2)/(1-x)^4, {x, 0, 50}], x] (* Vincenzo Librandi, Jul 04 2012 *)
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PROG
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(Magma) I:=[0, 9, 50, 147]; [n le 4 select I[n] else 4*Self(n-1)-6*Self(n-2)+4*Self(n-3)-Self(n-4): n in [1..40]]; // Vincenzo Librandi, Jul 04 2012
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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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