OFFSET
0,2
COMMENTS
Related identity: 0 = Sum_{n=-oo..+oo} x^n * (y - x^n)^n, which holds as a formal power series for all y.
LINKS
Paul D. Hanna, Table of n, a(n) for n = 0..300
FORMULA
The g.f. A(x) = Sum_{n>=0} a(n)*x^n satisfies:
(1.a) A(x) = Sum_{n=-oo..+oo} x^n * (3 + x^n)^(2*n).
(1.b) A(x) = Sum_{n=-oo..+oo} x^n * (3 - x^n)^(2*n).
(2.a) A(x) = Sum_{n=-oo..+oo} x^(2*n^2-n) / (1 - 3*x^n)^(2*n).
(2.b) A(x) = Sum_{n=-oo..+oo} x^(2*n^2-n) / (1 + 3*x^n)^(2*n).
(3.a) A(x^2) = (1/2) * Sum_{n=-oo..+oo} x^n * (3 + x^n)^n.
(3.b) A(x^2) = (1/2) * Sum_{n=-oo..+oo} x^n * (-3 + x^n)^n.
(4.a) A(x^2) = (1/2) * Sum_{n=-oo..+oo} x^(n^2-n) / (1 + 3*x^n)^n.
(4.b) A(x^2) = (1/2) * Sum_{n=-oo..+oo} x^(n^2-n) / (1 - 3*x^n)^n.
From Paul D. Hanna, Aug 06 2023: (Start)
The following generating functions are extensions of Peter Bala's formulas given in A260147.
(5.a) A(x^2) = Sum_{n=-oo..+oo} x^(2*n+1) * (3 + x^(2*n+1))^(2*n+1).
(5.b) A(x^2) = Sum_{n=-oo..+oo} x^(2*n*(2*n+1)) / (1 + 3*x^(2*n+1))^(2*n+1).
(End)
a(2^n) = 9^(2^n) for n > 0 (conjecture).
a(p) = p*3^(p-1) + 9^p for primes p > 3 (conjecture).
EXAMPLE
G.f.: A(x) = 1 + 10*x + 81*x^2 + 757*x^3 + 6561*x^4 + 59454*x^5 + 531496*x^6 + 4788072*x^7 + 43046721*x^8 + 387480753*x^9 + ...
PROG
(PARI) {a(n) = my(A); A = sum(m=-n-1, n+1, x^m * (3 + x^m +x*O(x^n))^(2*m) ); polcoeff(A, n)}
for(n=0, 30, print1(a(n), ", "))
CROSSREFS
KEYWORD
nonn
AUTHOR
Paul D. Hanna, Aug 01 2023
STATUS
approved