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A284444
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a(n) = Sum_{d|n, d == 3 (mod 7)} d.
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10
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0, 0, 3, 0, 0, 3, 0, 0, 3, 10, 0, 3, 0, 0, 3, 0, 17, 3, 0, 10, 3, 0, 0, 27, 0, 0, 3, 0, 0, 13, 31, 0, 3, 17, 0, 3, 0, 38, 3, 10, 0, 3, 0, 0, 48, 0, 0, 27, 0, 10, 20, 52, 0, 3, 0, 0, 3, 0, 59, 13, 0, 31, 3, 0, 0, 69, 0, 17, 3, 10, 0, 27, 73, 0, 3, 38, 0, 3, 0, 90, 3
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OFFSET
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1,3
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LINKS
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FORMULA
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G.f.: Sum_{k>=0} (7*k + 3)*x^(7*k+3)/(1 - x^(7*k+3)). - 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] == 3, 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) == 3)); \\ 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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