OFFSET
0,1
COMMENTS
a(n) == 0 (mod 3^2) for n > 0.
LINKS
Paul D. Hanna, Table of n, a(n) for n = 0..200
FORMULA
G.f. A(x) = Sum_{n>=0} a(n)*x^n may be defined by the following.
(1) 1/3 = Sum_{n=-oo..+oo} x^n * (A(x) - x^n)^(n-1).
(2) 1/3 = Sum_{n=-oo..+oo} (-1)^(n+1) * x^(n^2) / (1 - x^n*A(x))^(n+1).
(3) A(x)/3 = Sum_{n=-oo..+oo} x^(2*n) * (A(x) - x^n)^(n-1).
(4) A(x)/3 = Sum_{n=-oo..+oo} (-1)^(n+1) * x^(n*(n-1)) / (1 - x^n*A(x))^(n+1).
EXAMPLE
G.f.: A(x) = 4 + 18*x + 216*x^2 + 3006*x^3 + 46062*x^4 + 752058*x^5 + 12824370*x^6 + 225765756*x^7 + 4072115322*x^8 + 74865020256*x^9 + ...
PROG
(PARI) {a(n) = my(A=[4]); for(i=1, n, A = concat(A, 0);
A[#A] = polcoeff(-3 + 3^2*sum(m=-#A, #A, x^m * (Ser(A) - x^m)^(m-1) ), #A-1); ); A[n+1]}
for(n=0, 30, print1(a(n), ", "))
CROSSREFS
KEYWORD
nonn
AUTHOR
Paul D. Hanna, May 28 2023
STATUS
approved