OFFSET
2,2
LINKS
G. C. Greubel, Table of n, a(n) for n = 2..2500
FORMULA
Expansion of (eta(q) * eta(q^9)^2 / eta(q^3))^3 in powers of q.
Euler transform of period 9 sequence [-3, -3, 0, -3, -3, 0, -3, -3, -6, ...].
a(3*n + 1) = 0. a(3*n) = -3 * A106402(n).
EXAMPLE
G.f. = q^2 - 3*q^3 + 8*q^5 - 9*q^6 + 17*q^8 - 27*q^9 + 40*q^11 - 39*q^12 + ...
MATHEMATICA
eta[q_]:= q^(1/24)*QPochhammer[q]; CoefficientList[Series[(eta[q]* eta[q^9]^2/eta[q^3])^3, {q, 0, 50}], q] (* G. C. Greubel, Aug 11 2018 *)
PROG
(PARI) {a(n) = my(A); if( n<2, 0, n = n-2; A = x * O(x^n); polcoeff( (eta(x + A) * eta(x^9 + A)^2 / eta(x^3 + A))^3, n))};
CROSSREFS
KEYWORD
sign
AUTHOR
Michael Somos, Apr 04 2012
STATUS
approved