OFFSET
0,8
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^(-1/3) * eta(q) * eta(q^15) / (eta(q^3) * eta(q^5)) in powers of q.
Euler transform of period 15 sequence [ -1, -1, 0, -1, 0, 0, -1, -1, 0, 0, -1, 0, -1, -1, 0, ...].
Given g.f. A(x), then B(x) = x * A(x^3) satisfies 0 = f(B(x), B(x^2)) where f(u, v) = (v - u^2) * (u - v^2) - 2 * u^2 * v^2.
EXAMPLE
1 - x - x^2 + x^3 - x^4 + x^5 + x^6 - 2*x^7 + 2*x^9 - x^11 + x^12 + ...
q - q^4 - q^7 + q^10 - q^13 + q^16 + q^19 - 2*q^22 + 2*q^28 - q^34 + q^37 + ...
MATHEMATICA
eta[q_]:= q^(1/24)*QPochhammer[q]; a[n_]:= SeriesCoefficient[q^(-1/3)* eta[q]*eta[q^15]/(eta[q^3]*eta[q^5]), {q, 0, n}]; Table[a[n], {n, 0, 50}] (* G. C. Greubel, Apr 03 2018 *)
PROG
(PARI) {a(n) = local(A); if( n<0, 0, A = x * O(x^n); polcoeff( eta(x + A) * eta(x^15 + A) / (eta(x^3 + A) * eta(x^5 + A)), n))}
CROSSREFS
KEYWORD
sign
AUTHOR
Michael Somos, Aug 04 2011
STATUS
approved