OFFSET
0,3
COMMENTS
Compare to g.f. of partition numbers: exp( Sum_{n>=1} sigma(n)*x^n/n ), where sigma(n) = A000203(n) is the sum of the divisors of n.
LINKS
Seiichi Manyama, Table of n, a(n) for n = 0..335
FORMULA
a(n) = (1/n)*Sum_{k=1..n} sigma(k)^(k-1) * a(n-k) for n > 0, with a(0)=1.
Logarithmic derivative yields A224440.
EXAMPLE
G.f.: A(x) = 1 + x + 2*x^2 + 7*x^3 + 93*x^4 + 357*x^5 + 41927*x^6 + ... where
log(A(x)) = x + 3^1*x^2/2 + 4^2*x^3/3 + 7^3*x^4/4 + 6^4*x^5/5 + 12^5*x^6/6 + ...
PROG
(PARI) {a(n)=polcoeff(exp(sum(m=1, n, sigma(m)^(m-1)*x^m/m)+x*O(x^n)), n)}
for(n=0, 20, print1(a(n), ", "))
(PARI) {a(n)=if(n==0, 1, (1/n)*sum(k=1, n, sigma(k)^(k-1)*a(n-k)))}
CROSSREFS
KEYWORD
nonn
AUTHOR
Paul D. Hanna, Apr 06 2013
STATUS
approved