OFFSET
1,2
FORMULA
E.g.f.: Sum_{k>=1} (exp(2*x^k) - 1) / 2.
a(p) = p! + 2^(p - 1), where p is prime.
MATHEMATICA
Table[n! Sum[2^(d - 1)/d!, {d, Divisors[n]}], {n, 1, 22}]
nmax = 22; CoefficientList[Series[Sum[(Exp[2 x^k] - 1)/2, {k, 1, nmax}], {x, 0, nmax}], x] Range[0, nmax]! // Rest
PROG
(PARI) a(n) = n! * sumdiv(n, d, 2^(d-1)/d!); \\ Michel Marcus, Aug 12 2020
CROSSREFS
KEYWORD
nonn
AUTHOR
Ilya Gutkovskiy, Aug 10 2020
STATUS
approved