OFFSET
1,2
FORMULA
G.f.: Sum_{k>0} k * x^k / (1 - 2 * k* x^k).
MATHEMATICA
a[n_] := DivisorSum[n, (2*#)^(n/#) &] / 2; Array[a, 33] (* Amiram Eldar, Aug 14 2023 *)
PROG
(PARI) a(n) = sumdiv(n, d, (2*d)^(n/d))/2;
(PARI) my(N=40, x='x+O('x^N)); Vec(sum(k=1, N, k*x^k/(1-2*k*x^k)))
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Seiichi Manyama, Jan 12 2023
STATUS
approved