OFFSET
0,4
COMMENTS
LINKS
G. C. Greubel, Table of n, a(n) for n = 0..1000
Christian Kassel, Christophe Reutenauer, The Fourier expansion of eta(z)eta(2z)eta(3z)/eta(6z), arXiv:1603.06357 [math.NT], 2016.
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^3) * eta(q^6) / eta(q^2) in powers of q.
Euler transform of period 6 sequence [ -1, 0, -2, 0, -1, -2, ...].
G.f.: Product_{k>0} (1 - x^(3*k)) * (1 - x^(6*k)) / (1 + x^k).
a(n) = (-1)^n * A122865(n).
a(2*n + 1) = - A122856(n). a(4*n) = A002175(n). a(4*n + 1) = - A122865(n). a(4*n + 2) = a(8*n + 7) = 0. a(8*n + 3) = -2 * A121444(n). a(8*n + 5) = - A122856(n).
a(n) = -A258210(3*n + 1). - Michael Somos, May 01 2016
EXAMPLE
G.f. = 1 - x - 2*x^3 + 2*x^4 - x^5 + 3*x^8 - 2*x^11 + 2*x^12 - 2*x^13 + ...
G.f. = q - q^4 - 2*q^10 + 2*q^13 - q^16 + 3*q^25 - 2*q^34 + 2*q^37 + ...
MATHEMATICA
a[ n_] := SeriesCoefficient[ QPochhammer[ x^3] QPochhammer[ x^6] / QPochhammer[ -x, x], {x, 0, n}];
PROG
(PARI) {a(n) = my(A); if( n<0, 0, A = x * O(x^n); polcoeff( eta(x + A) * eta(x^3 + A) * eta(x^6 + A) / eta(x^2 + A), n))};
CROSSREFS
KEYWORD
sign
AUTHOR
Michael Somos, May 25 2015
STATUS
approved