OFFSET
0,2
FORMULA
G.f. A(x) satisfies the following identities.
(1) 1 = Sum_{n>=0} ( (1+x)^n - 7*x*A(x) )^n * 3^n / 4^(n+1).
(2) 1 = Sum_{n>=0} (1+x)^(n^2) * 3^n / (4 + 21*x*A(x)*(1+x)^n)^(n+1).
EXAMPLE
G.f.: A(x) = 1 + 27*x + 2625*x^2 + 429195*x^3 + 95328009*x^4 + 26290301175*x^5 + 8582072887881*x^6 + 3220902003386403*x^7 + 1363088948866736193*x^8 + ...
such that
1 = 1/4 + ((1+x) - 7*x*A(x))*3/4^2 + ((1+x)^2 - 7*x*A(x))^2*3^2/4^3 + ((1+x)^3 - 7*x*A(x))^3*3^3/4^4 + ((1+x)^4 - 7*x*A(x))^4*3^4/4^5 + ...
Also,
1 = 1/(4 + 21*x*A(x)) + (1+x)*3/(4 + 21*x*A(x)*(1+x))^2 + (1+x)^4*3^2/(4 + 21*x*A(x)*(1+x)^2)^3 + (1+x)^9*3^3/(4 + 21*x*A(x)*(1+x)^3)^4 + ...
PROG
(PARI) \p120
{A=vector(1); A[1]=1; for(i=1, 20, A = concat(A, 0);
A[#A] = round( Vec( sum(n=0, 1200, ( (1+x +x*O(x^#A))^n - 7*x*Ser(A) )^n * 3^n/4^(n+1)*1.)/21 ) )[#A+1]); A}
CROSSREFS
KEYWORD
nonn
AUTHOR
Paul D. Hanna, Jan 10 2019
STATUS
approved