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^(1/8) * eta(q^4)^4 * eta(q) / (eta(q^2)^2 * eta(q^8)^2) in powers of q.
Euler transform of period 8 sequence [ -1, 1, -1, -3, -1, 1, -1, -1, ...].
G.f. is a period 1 Fourier series which satisfies f(-1 / (256 t)) = 32^(1/2) (t/i)^(1/2) g(t) where q = exp(2 Pi i t) and g() is the g.f. for A132965.
G.f.: Product_{k>0} (1 - x^k) * (1 + x^(2*k))^2 / (1 + x^(4*k))^2.
EXAMPLE
G.f. = 1 - x + x^2 - 2*x^3 - x^4 + x^6 + x^7 + 2*x^8 - x^9 - x^11 + x^13 + ...
G.f. = q^-1 - q^7 + q^15 - 2*q^23 - q^31 + q^47 + q^55 + 2*q^63 - q^71 - ...
MATHEMATICA
a[ n_] := SeriesCoefficient[ QPochhammer[ x] QPochhammer[ -x^2, x^4]^2, {x, 0, n}]; (* Michael Somos, Oct 31 2015 *)
PROG
(PARI) {a(n) = my(A); if( n<0, 0, A = x * O(x^n); polcoeff( eta(x^4 + A)^4 * eta(x + A) / (eta(x^2 + A)^2 * eta(x^8 + A)^2), n))};
CROSSREFS
KEYWORD
sign
AUTHOR
Michael Somos, Aug 23 2007
STATUS
approved