A350124 a(n) = Sum_{k=1..n} k^2 * floor(n/k)^3.

1, 12, 40, 121, 207, 473, 649, 1142, 1611, 2401, 2853, 4647, 5285, 6879, 8759, 11452, 12558, 16739, 18127, 23353, 27129, 31171, 33219, 43573, 47524, 53210, 59538, 69996, 73274, 89694, 93446, 107195, 116731, 126545, 137505, 164580, 169946, 182244, 195644, 225454

Offset

1,2

Links

Seiichi Manyama, Table of n, a(n) for n = 1..10000

Formula

a(n) = Sum_{k=1..n} k^2 * Sum_{d|k} (d^3 - (d - 1)^3)/d^2.

G.f.: (1/(1 - x)) * Sum_{k>=1} (k^3 - (k - 1)^3) * x^k * (1 + x^k)/(1 - x^k)^3.

From Vaclav Kotesovec, Aug 03 2022: (Start)

a(n) = A064602(n) - 3*A143128(n) + 3*A319085(n).

a(n) ~ n^3 * (log(n) + 2*gamma + (zeta(3) - 1)/3 - Pi^2/6), where gamma is the Euler-Mascheroni constant A001620. (End)

Mathematica

Accumulate[Table[DivisorSigma[2, k] - 3*k*DivisorSigma[1, k] + 3*k^2*DivisorSigma[0, k], {k, 1, 50}]] (* Vaclav Kotesovec, Dec 17 2021 *)

Prog

(PARI) a(n) = sum(k=1, n, k^2*(n\k)^3);
(PARI) a(n) = sum(k=1, n, k^2*sumdiv(k, d, (d^3-(d-1)^3)/d^2));
(PARI) my(N=66, x='x+O('x^N)); Vec(sum(k=1, N, (k^3-(k-1)^3)*x^k*(1+x^k)/(1-x^k)^3)/(1-x))
(Python)
from math import isqrt
def A350124(n): return (-(s:=isqrt(n))**4*(s+1)*(2*s+1) + sum((q:=n//k)*(k*(3*(k-1))+q*(k*(9*(k-1))+q*(k*(12*k-6)+2)+3)+1) for k in range(1, s+1)))//6 # Chai Wah Wu, Oct 21 2023

Crossrefs

Cf. A318742, A350108, A350123, A350125.

Cf. A064602, A143128, A319085.

Sequence in context: A033586 A211786 A320252 * A359566 A352144 A180093

Adjacent sequences: A350121 A350122 A350123 * A350125 A350126 A350127

Keyword

nonn

Author

Seiichi Manyama, Dec 15 2021

Status

approved