OFFSET
0,3
FORMULA
G.f. A(x) satisfies:
(1) x*A(x) = Sum_{n=-oo..+oo} (-1)^n * x^(n*(n+1)) * A(x)^n.
(2) -x*A(x)^2 = Sum_{n=-oo..+oo} (-1)^n * x^(n*(n+1)) / A(x)^n.
(3) x*A(x) = Product_{n>=1} (1 - x^(2*n)*A(x)) * (1 - x^(2*n-2)/A(x)) * (1 - x^(2*n)), due to the Jacobi triple product identity.
(4) -x*A(x)^2 = Product_{n>=1} (1 - x^(2*n)/A(x)) * (1 - x^(2*n-2)*A(x)) * (1 - x^(2*n)), due to the Jacobi triple product identity.
EXAMPLE
G.f.: A(x) = 1 + x + 2*x^2 + 8*x^3 + 26*x^4 + 97*x^5 + 361*x^6 + 1399*x^7 + 5532*x^8 + 22318*x^9 + 91387*x^10 + 379037*x^11 + 1588769*x^12 + ...
such that
x*A(x) = ... + x^12/A(x)^4 - x^6/A(x)^3 + x^2/A(x)^2 - 1/A(x) + 1 - x^2*A(x) + x^6*A(x)^2 - x^12*A(x)^3 + x^20*A(x)^4 + ... + (-1)^n * x^(n*(n+1)) * A(x)^n + ...
PROG
(PARI) {a(n, p=1) = my(A=[1]); for(i=1, n, A=concat(A, 0);
A[#A] = polcoeff( x*Ser(A)^p - sum(m=-ceil(sqrt(n+1)), ceil(sqrt(n+1)), (-1)^m*x^(m*(m+1))*Ser(A)^m ), #A-1)); A[n+1]}
for(n=0, 30, print1(a(n), ", "))
CROSSREFS
KEYWORD
nonn
AUTHOR
Paul D. Hanna, Sep 18 2022
STATUS
approved