OFFSET
1,2
FORMULA
G.f.: Sum_{k>0} (2 * k)^(k-1) * x^k / (1 - x^k).
MATHEMATICA
a[n_] := DivisorSum[n, (2*#)^(# - 1) &]; Array[a, 20] (* Amiram Eldar, Aug 14 2023 *)
PROG
(PARI) a(n) = sumdiv(n, d, (2*d)^(d-1));
(PARI) my(N=20, x='x+O('x^N)); Vec(sum(k=1, N, (2*k)^(k-1)*x^k/(1-x^k)))
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Seiichi Manyama, Jan 13 2023
STATUS
approved