OFFSET
0,4
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^(-5/8) * eta(q) * eta(q^12)^2 / (eta(q^2)^2 * eta(q^6)) in powers of q.
Euler transform of period 12 sequence [ -1, 1, -1, 1, -1, 2, -1, 1, -1, 1, -1, 0, ...].
a(n) = (-1)^n * A132217(n).
Product_{k>0} (1 - x^(12*k)) * (1 - x^(2*k) + x^(4*k)) / (1 - (-x)^k). - Michael Somos, Oct 04 2015
EXAMPLE
G.f. = 1 - x + x^2 - 2*x^3 + 3*x^4 - 4*x^5 + 6*x^6 - 8*x^7 + 11*x^8 + ...
G.f. = q^5 - q^13 + q^21 - 2*q^29 + 3*q^37 - 4*q^45 + 6*q^53 - 8*q^61 + ...
MATHEMATICA
a[ n_] := SeriesCoefficient[ x^(-5/8) EllipticTheta[ 2, 0, x^3] / EllipticTheta[ 2, 0, x^(1/2)], {x, 0, n}];
PROG
(PARI) {a(n) = my(A); if( n<0, 0, A = x * O(x^n); polcoeff( eta(x + A) * eta(x^12 + A)^2 / (eta(x^2 + A)^2 * eta(x^6 + A)), n))};
CROSSREFS
KEYWORD
sign
AUTHOR
Michael Somos, Sep 13 2015
STATUS
approved