OFFSET
1,2
FORMULA
G.f.: Sum_{k >= 1} (1/(1 - (k * x)^k)^k - 1).
If p is prime, a(p) = 1 + p^(p+1).
MATHEMATICA
a[n_] := DivisorSum[n, (n/#)^n * Binomial[# + n/# - 1, #] &]; Array[a, 20] (* Amiram Eldar, Apr 22 2021 *)
PROG
(PARI) a(n) = sumdiv(n, d, (n/d)^n*binomial(d+n/d-1, d));
(PARI) N=20; x='x+O('x^N); Vec(sum(k=1, N, 1/(1-(k*x)^k)^k-1))
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, Apr 22 2021
STATUS
approved