OFFSET
0,3
FORMULA
G.f. A(x) = Sum_{n>=0} a(n)*x^n satisfies:
(1) 1 = Sum_{n>=0} (-x)^n * A(x)^(2*n) * A(x*A(x)^n),
(2) 1 = Sum_{n>=0} a(n) * x^n / (1 + x*A(x)^(n+2)).
EXAMPLE
G.f.: A(x) = 1 + x + 2*x^2 + 6*x^3 + 23*x^4 + 103*x^5 + 516*x^6 + 2819*x^7 + 16517*x^8 + 102615*x^9 + 670503*x^10 + 4580064*x^11 + ...
where
(1) 1 = A(x) - x*A(x)^2*A(x*A(x)) + x^2*A(x)^4*A(x*A(x)^2) - x^3*A(x)^6*A(x*A(x)^3) + x^4*A(x)^8*A(x*A(x)^4) - x^5*A(x)^10*A(x*A(x)^5) + x^6*A(x)^12*A(x*A(x)^6) + ...
(2) 1 = 1/(1 + x*A(x)^2) + 1*x/(1 + x*A(x)^3) + 2*x^2/(1 + x*A(x)^4) + 6*x^3/(1 + x*A(x)^5) + 23*x^4/(1 + x*A(x)^6) + 103*x^5/(1 + x*A(x)^7) + 516*x^6/(1 + x*A(x)^8) + ... + a(n)*x^n/(1 + x*A(x)^(n+2)) + ...
PROG
(PARI) /* 1 = Sum_{n>=0} (-x)^n * A(x)^(2*n) * A(x*A(x)^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)^(2*n)*subst(Ser(A), x, x*Ser(A)^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)^(n+2)) */
{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)^(n+2)) ), #A-1)); A[n+1]}
for(n=0, 31, print1(a(n), ", "))
CROSSREFS
KEYWORD
nonn
AUTHOR
Paul D. Hanna, Apr 05 2022
STATUS
approved