OFFSET
0,2
COMMENTS
LINKS
G. C. Greubel, Table of n, a(n) for n = 0..1000
Michael Somos, Introduction to Ramanujan theta functions
Eric Weisstein's World of Mathematics, Ramanujan Theta Functions
FORMULA
Expansion of f(x^3) * a(-x) in powers of x where f() is a Ramanujan theta function and a() is a cubic AGM theta function.
a(n) = b(8*n + 1) where b() is multiplicative with b(2^e) = 0^e, b(3^e) = - (1 + (-1)^e) * 3^(e/2), b(p^e) = (1 + (-1)^e)/2 * p^(e/2) if p == 1, 3 (mod 8), b(p^e) = (1 + (-1)^e)/2 * (-p)^(e/2) if p == 5, 7 (mod 8). - Michael Somos, Jun 19 2015
G.f. is a period 1 Fourier series which satisfies f(-1 / (2304 t)) = -576 (t/i)^(3/2) g(t) where q = exp(2 Pi i t) and g() is the g.f. for A133079.
EXAMPLE
G.f. = 1 - 6*x - 5*x^3 - 7*x^6 - 18*x^10 + 11*x^15 - 13*x^21 + 30*x^28 + ...
G.f. = q - 6*q^9 - 5*q^25 - 7*q^49 - 18*q^81 + 11*q^121 - 13*q^169 + ...
MATHEMATICA
a[ n_] := With[ {m = Sqrt[8 n + 1]}, If[ IntegerQ@m, m (-1)^(n + Quotient[m, 6]), 0] If[ Divisible[ m, 3], 2, 1]]; (* Michael Somos, Jun 19 2015 *)
a[ n_] := SeriesCoefficient[ QPochhammer[ -x]^3 - 9 x QPochhammer[ -x^9]^3, {x, 0, n}]; (* Michael Somos, Jun 19 2015 *)
PROG
(PARI) {a(n) = my(m); if( issquare(8*n + 1, &m), (-1)^(m \ 6 + n) * m * ((m%3 == 0) + 1), 0)};
(PARI) {a(n) = my(A); if( n<0, 0, A = x * O(x^n); polcoeff( eta(-x + A)^3 - 9 * x * eta(-x^9 + A)^3, n))};
CROSSREFS
KEYWORD
sign
AUTHOR
Michael Somos, Jan 19 2012
STATUS
approved