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A284443
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a(n) = Sum_{d|n, d == 2 (mod 7)} d.
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10
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0, 2, 0, 2, 0, 2, 0, 2, 9, 2, 0, 2, 0, 2, 0, 18, 0, 11, 0, 2, 0, 2, 23, 2, 0, 2, 9, 2, 0, 32, 0, 18, 0, 2, 0, 11, 37, 2, 0, 2, 0, 2, 0, 46, 9, 25, 0, 18, 0, 2, 51, 2, 0, 11, 0, 2, 0, 60, 0, 32, 0, 2, 9, 18, 65, 2, 0, 2, 23, 2, 0, 83, 0, 39, 0, 2, 0, 2, 79, 18, 9
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OFFSET
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1,2
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LINKS
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FORMULA
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G.f.: Sum_{k>=0} (7*k + 2)*x^(7*k+2)/(1 - x^(7*k+2)). - Ilya Gutkovskiy, Mar 28 2017
Sum_{k=1..n} a(k) = c * n^2 + O(n*log(n)), where c = Pi^2/84 = 0.117495... . - Amiram Eldar, Nov 26 2023
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MATHEMATICA
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Table[Sum[If[Mod[d, 7] == 2, d, 0], {d, Divisors[n]}], {n, 80}] (* Indranil Ghosh, Mar 27 2017 *)
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PROG
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(PARI) a(n) = sumdiv(n, d, d*((d % 7) == 2)); \\ Amiram Eldar, Nov 26 2023
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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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