OFFSET
1,2
LINKS
Seiichi Manyama, Table of n, a(n) for n = 1..10000
FORMULA
a(n) = Sum_{k=1,..n} Sum_{d|k} (-1)^(k/d + 1) * (2*d - 1).
G.f.: (1/(1 - x)) * Sum_{k>=1} (2*k - 1) * x^k/(1 + x^k).
a(n) ~ Pi^2 * n^2 / 12. - Vaclav Kotesovec, May 28 2021
MATHEMATICA
a[n_] := Sum[(-1)^(k + 1) * Quotient[n, k]^2, {k, 1, n}]; Array[a, 50] (* Amiram Eldar, May 27 2021 *)
Accumulate[Table[-2*DivisorSigma[0, 2*n] + 3*DivisorSigma[0, n] + 2*DivisorSigma[1, 2*n] - 4*DivisorSigma[1, n], {n, 1, 50}]] (* Vaclav Kotesovec, May 28 2021 *)
PROG
(PARI) a(n) = sum(k=1, n, (-1)^(k+1)*(n\k)^2);
(PARI) a(n) = sum(k=1, n, sumdiv(k, d, (-1)^(k/d+1)*(2*d-1)));
(PARI) my(N=66, x='x+O('x^N)); Vec(sum(k=1, N, (2*k-1)*x^k/(1+x^k))/(1-x))
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, May 27 2021
STATUS
approved