OFFSET
0,2
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
Euler transform of period 12 sequence [ 6, -12, 0, -6, 6, 0, 6, -6, 0, -12, 6, 0, ...].
G.f. is a period 1 Fourier series which satisfies f(-1 / (48 t)) = 27 g(t) where q = exp(2 Pi i t) and g() is the g.f. for A264026.
G.f.: (Product_{k>0} (1 + x^k) * (1 - x^(2*k)) * (1 + x^(3*k)) * (1 - x^(6*k)) / ((1 + x^(2*k)) * (1 + x^(6*k))))^6.
EXAMPLE
G.f. = 1 + 6*x + 9*x^2 - 16*x^3 - 66*x^4 - 54*x^5 + 98*x^6 + 300*x^7 + 243*x^8 + ...
G.f. = 1/q + 6*q + 9*q^3 - 16*q^5 - 66*q^7 - 54*q^9 + 98*q^11 + 300*q^13 + ...
MATHEMATICA
a[ n_] := SeriesCoefficient[ (QPochhammer[ -x] / QPochhammer[ -x^3])^6, {x, 0, n}]; (* Michael Somos, Aug 26 2015 *)
PROG
(PARI) {a(n) = my(A); if( n<0, 0, A = x * O(x^n); polcoeff( (eta(x^2 + A)^3 * eta(x^3 + A) * eta(x^12 + A) / (eta(x + A) * eta(x^4 + A) * eta(x^6 + A)^3))^6, n))};
CROSSREFS
KEYWORD
sign
AUTHOR
Michael Somos, Aug 09 2007
STATUS
approved