OFFSET
1,2
FORMULA
G.f.: Sum_{k>0} (k-1)! * x^k/(1 - x^k).
If p is prime, a(p) = 1 + (p-1)!.
MATHEMATICA
a[n_] := DivisorSum[n, (# - 1)! &]; Array[a, 23] (* Amiram Eldar, Aug 30 2023 *)
PROG
(PARI) a(n) = sumdiv(n, d, (d-1)!);
(PARI) my(N=30, x='x+O('x^N)); Vec(sum(k=1, N, (k-1)!*x^k/(1-x^k)))
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Seiichi Manyama, Nov 08 2022
STATUS
approved