OFFSET
1,2
FORMULA
G.f.: Sum_{k>=1} k * x^k/(1 - x^k)^5 = Sum_{k>=1} binomial(k+3,4) * x^k/(1 - x^k)^2.
MATHEMATICA
a[n_] := n * DivisorSum[n, Binomial[# + 3, 4]/# &]; Array[a, 50] (* Amiram Eldar, Apr 25 2021 *)
PROG
(PARI) a(n) = n*sumdiv(n, d, binomial(d+3, 4)/d);
(PARI) my(N=66, x='x+O('x^N)); Vec(sum(k=1, N, binomial(k+3, 4)*x^k/(1-x^k)^2))
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, Apr 19 2021
STATUS
approved