OFFSET
1,2
EXAMPLE
G.f.: A(x) = x + 2*x^2 + 12*x^3 + 264*x^4 + 18192*x^5 + 3363168*x^6 +...
where
A(A(x)) = x/(1-2*x) + 2*x^2/(1-4*x)^2 + 12*x^3/(1-8*x)^3 + 264*x^4/(1-16*x)^4 + 18192*x^5/(1-32*x)^5 +...+ a(n)*x^n/(1-2^n*x)^n +...
Explicitly,
A(A(x)) = x + 4*x^2 + 32*x^3 + 656*x^4 + 40224*x^5 + 7013760*x^6 +...
PROG
(PARI) {a(n)=local(A=[1], F=x, G=x); for(i=1, n, A=concat(A, 0); F=x*Ser(A);
G=sum(m=1, #A-1, A[m]*x^m/(1-2^m*x+x*O(x^#A))^m);
A[#A]=Vec(G)[#A]-Vec(subst(F, x, F))[#A]); if(n<1, 0, A[n])}
CROSSREFS
KEYWORD
nonn
AUTHOR
Paul D. Hanna, Jul 19 2011
STATUS
approved