OFFSET
0,2
COMMENTS
LINKS
Michael Somos, Introduction to Ramanujan theta functions
Eric Weisstein's World of Mathematics, Ramanujan Theta Functions
FORMULA
Expansion of (chi(q) * chi(q^3))^3 - 8 * q / (chi(q) * chi(q^3))^3 in powers of q where chi() is a Ramanujan theta function.
G.f. is a period 1 Fourier series which satisfies f(-1 / (48 t)) = f(t) where q = exp(2 Pi i t).
a(n) = (-1)^n * A058490(n).
EXAMPLE
G.f. = 1 - 5*x + 27*x^2 - 41*x^3 + 146*x^4 - 243*x^5 + 5120*x^6 + ...
G.f. = 1/q - 5*q + 27*q^3 - 41*q^5 + 146*q^7 - 243*q^9 + 510*q^11 - 887*q^13 + ...
PROG
(PARI) {a(n) = my(A); if( n<0, 0, A = x * O(x^n); A = ((eta(x^2 + A) * eta(x^6 + A))^2 / (eta(x + A) * eta(x^3 + A) * eta(x^4 + A) * eta(x^12 + A)))^3; polcoeff( A - 8 * x / A, n))};
CROSSREFS
KEYWORD
sign
AUTHOR
Michael Somos, Feb 06 2009
STATUS
approved