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A344723
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a(n) = Sum_{k=1..n} (-1)^(k+1) * floor(n/k)^5.
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2
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1, 31, 243, 992, 3094, 7564, 16596, 31744, 58237, 97117, 158169, 241837, 364299, 521829, 745693, 1018120, 1389402, 1837302, 2423834, 3105432, 3998776, 5007286, 6289998, 7738784, 9543887, 11537207, 14031231, 16717879, 20018661, 23629281, 27958433, 32577739, 38219963, 44148743
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OFFSET
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1,2
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COMMENTS
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In general, for m > 1, Sum_{k=1..n} (-1)^(k+1) * floor(n/k)^m ~ (1 - 2^(1-m)) * zeta(m) * n^m. - Vaclav Kotesovec, May 28 2021
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LINKS
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FORMULA
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a(n) = Sum_{k=1,..n} Sum_{d|k} (-1)^(k/d + 1) * (d^5 - (d - 1)^5).
G.f.: (1/(1 - x)) * Sum_{k>=1} (k^5 - (k - 1)^5) * x^k/(1 + x^k).
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MATHEMATICA
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a[n_] := Sum[(-1)^(k + 1) * Quotient[n, k]^5, {k, 1, n}]; Array[a, 50] (* Amiram Eldar, May 27 2021 *)
Accumulate[Table[-3*DivisorSigma[0, n] + 2*DivisorSigma[0, 2*n] + 10*DivisorSigma[1, n] - 5*DivisorSigma[1, 2*n] - 15*DivisorSigma[2, n] + 5*DivisorSigma[2, 2*n] + 25/2 * DivisorSigma[3, n] - 5/2 * DivisorSigma[3, 2*n] - 45/8 *DivisorSigma[4, n] + 5/8 * DivisorSigma[4, 2*n], {n, 1, 50}]] (* Vaclav Kotesovec, May 28 2021 *)
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PROG
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(PARI) a(n) = sum(k=1, n, (-1)^(k+1)*(n\k)^5);
(PARI) a(n) = sum(k=1, n, sumdiv(k, d, (-1)^(k/d+1)*(d^5-(d-1)^5)));
(PARI) my(N=40, x='x+O('x^N)); Vec(sum(k=1, N, (k^5-(k-1)^5)*x^k/(1+x^k))/(1-x))
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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