OFFSET
0,6
FORMULA
Euler transform of period 20 sequence [ 0, 0, 0, 1, -2, 0, 0, 1, 0, 0, 0, 1, 0, 0, -2, 1, 0, 0, 0, 0, ...].
EXAMPLE
q + q^17 - 2*q^21 + 2*q^33 - 2*q^37 + q^41 + 3*q^49 - 4*q^53 + q^57 + ...
MATHEMATICA
QP = QPochhammer; s = QP[q^5]^2*(QP[q^20]/(QP[q^4]*QP[q^10]^2)) + O[q]^80; CoefficientList[s, q] (* Jean-François Alcover, Nov 25 2015 *)
PROG
(PARI) {a(n) = local(A); if( n<0, 0, A = x * O(x^n); polcoeff( eta(x^5 + A)^2 * eta(x^20 + A) / (eta(x^4 + A) * eta(x^10 + A)^2), n))}
CROSSREFS
KEYWORD
sign
AUTHOR
Michael Somos, Oct 27 2008
STATUS
approved