OFFSET
1,2
COMMENTS
Compare g.f. to that of A000081 (number of rooted trees with n nodes):
* G(x) = x*exp( Sum_{n>=1} G(x^n)/n ).
EXAMPLE
G.f.: A(x) = 1 + 2*x + 18*x^2 + 348*x^3 + 22998*x^4 + 7497180*x^5 +...
A(x) = x*exp(A(2*x) + A(2^4*x^2)/2 + A(2^9*x^3)/3 + A(2^16*x^4)/4 +...).
PROG
(PARI) {a(n)=local(A=x+2*x^2); for(i=1, n, A=x*exp(sum(m=1, n, subst(A, x, 2^(m^2)*x^m+x*O(x^n))/m))); polcoeff(A, n)}
CROSSREFS
KEYWORD
nonn
AUTHOR
Paul D. Hanna, Oct 30 2010
STATUS
approved