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 q^(-1/2) * ( (eta(q) * eta(q^3))^3 + 3 * (eta(q^3) * eta(q^9))^3 ) in powers of q^3.
a(n) = b(6*n + 1) where b(n) is multiplicative with b(2^e) = b(3^e) = 0^e, b(p^e) = p^e * (1 + (-1)^e) / 2 if p == 5 (mod 6), b(p^e) = b(p) * b(p^(e-1)) - p^2 * b(p^(e-2)) if p == 1 (mod 6).
G.f. is a period 1 Fourier series which satisfies f(-1 / (36 t)) = 15552^(1/2) (t / i)^3 g(t) where q = exp(2 Pi i t) and g() is the g.f. for A152244.
EXAMPLE
G.f. = 1 + 2*x - 22*x^2 + 26*x^3 + 25*x^4 - 46*x^5 + 26*x^6 - 22*x^7 + ...
G.f. = q + 2*q^7 - 22*q^13 + 26*q^19 + 25*q^25 - 46*q^31 + 26*q^37 - 22*q^43 + ...
MATHEMATICA
eta[q_]:= q^(1/24)*QPochhammer[q]; a:= CoefficientList[Series[q^(-1/6)*((eta[q^(1/3)]*eta[q^1])^3 + 3*(eta[q^1]*eta[q^3])^3), {q, 0, 60}], q]; Table[a[[n]], {n, 1, 50}] (* G. C. Greubel, Jun 10 2018 *)
PROG
(PARI) {a(n) = local(A); if( n<0, 0, n *= 3; A = x * O(x^n); polcoeff( (eta(x + A) * eta(x^3 + A))^3 + 3 * x * (eta(x^3 + A) * eta(x^9 + A))^3, n))};
CROSSREFS
KEYWORD
sign
AUTHOR
Michael Somos, Nov 30 2008
STATUS
approved