OFFSET
0,4
COMMENTS
LINKS
G. C. Greubel, Table of n, a(n) for n = 0..2500
Michael Somos, Introduction to Ramanujan theta functions
Eric Weisstein's World of Mathematics, Ramanujan Theta Functions
FORMULA
Expansion of f(x, x^2) / psi(x) in powers of x where psi(), f() are Ramanujan theta functions.
Expansion of q^(1/12) * eta(q^3)^2 / (eta(q^2) * eta(q^6)) in powers of q.
Euler transform of period 6 sequence [ 0, 1, -2, 1, 0, 0, ...].
G.f.: Product_{k>0} (1 + x^k + x^(2*k)) / ((1 + x^k) * (1 + x^(3*k))). [corrected by Vaclav Kotesovec, Jul 11 2016]
a(n) = (-1)^n * A258327(n). - Michael Somos, Jun 06 2015
EXAMPLE
G.f. = 1 + x^2 - 2*x^3 + 2*x^4 - 2*x^5 + 3*x^6 - 4*x^7 + 5*x^8 - 6*x^9 + ...
G.f. = 1/q + q^23 - 2*q^35 + 2*q^47 - 2*q^59 + 3*q^71 - 4*q^83 + 5*q^95 + ...
MATHEMATICA
a[ n_] := SeriesCoefficient[ EllipticTheta[ 4, 0, x] / QPochhammer[ x^2], {x, 0, n}];
a[ n_] := SeriesCoefficient[ QPochhammer[ x^3]^2 / (QPochhammer[ x^2] QPochhammer[ x^6]), {x, 0, n}]; (* Michael Somos, Jun 06 2015 *)
a[ n_] := SeriesCoefficient[ 2 x^(1/8) EllipticTheta[ 4, 0, x^3] / (EllipticTheta[ 2, 0, x^(1/2)] QPochhammer[ x, x^2]), {x, 0, n}]; (* Michael Somos, Jun 06 2015 *)
PROG
(PARI) {a(n) = my(A); if( n<0, 0, A = x * O(x^n); polcoeff( eta(x^3 + A)^2 / (eta(x^2 + A) * eta(x^6 + A)), n))};
CROSSREFS
KEYWORD
sign
AUTHOR
Michael Somos, Apr 06 2015
STATUS
approved