OFFSET
0,4
FORMULA
G.f. A(x) = Sum_{n>=0} a(n)*x^n satisfies:
(1) 1 = Sum_{n>=0} (-x)^n * A(x)^n * A(x*A(x)^(2*n)),
(2) 1 = Sum_{n>=0} a(n) * x^n / (1 + x*A(x)^(2*n+1)).
EXAMPLE
G.f.: A(x) = 1 + x + x^2 + 3*x^3 + 10*x^4 + 42*x^5 + 216*x^6 + 1208*x^7 + 7476*x^8 + 50476*x^9 + 365155*x^10 + 2809512*x^11 + ...
where
(1) 1 = A(x) - x*A(x)*A(x*A(x)^2) + x^2*A(x)^2*A(x*A(x)^4) - x^3*A(x)^3*A(x*A(x)^6) + x^4*A(x)^4*A(x*A(x)^8) - x^5*A(x)^5*A(x*A(x)^10) + x^6*A(x)^6*A(x*A(x)^12) + ...
(2) 1 = 1/(1 + x*A(x)) + 1*x/(1 + x*A(x)^3) + 1*x^2/(1 + x*A(x)^5) + 3*x^3/(1 + x*A(x)^7) + 10*x^4/(1 + x*A(x)^9) + 42*x^5/(1 + x*A(x)^11) + 216*x^6/(1 + x*A(x)^13) + ... + a(n)*x^n/(1 + x*A(x)^(2*n+1)) + ...
PROG
(PARI) /* 1 = Sum_{n>=0} (-x)^n * A(x)^n * A(x*A(x)^(2*n)) */
{a(n) = my(A=[1]); for(i=1, n, A=concat(A, 0);
A[#A] = -polcoeff( sum(n=0, #A-1, (-x)^n*Ser(A)^n*subst(Ser(A), x, x*Ser(A)^(2*n)) ), #A-1)); A[n+1]}
for(n=0, 31, print1(a(n), ", "))
(PARI) /* 1 = Sum_{n>=0} a(n) * x^n / (1 + x*A(x)^(2*n+1)) */
{a(n) = my(A=[1]); for(i=1, n, A=concat(A, 0);
A[#A] = -polcoeff( sum(n=0, #A-1, A[n+1]*x^n/(1 + x*Ser(A)^(2*n+1)) ), #A-1)); A[n+1]}
for(n=0, 31, print1(a(n), ", "))
CROSSREFS
KEYWORD
nonn
AUTHOR
Paul D. Hanna, Apr 05 2022
STATUS
approved