OFFSET
0,2
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 psi(x)^5 * f(-x)^3 = psi(x)^2 * f(-x^2)^6 in powers of x where psi(), f() are Ramanujan theta functions.
Expansion of q^(-3/4) * (eta(q^2)^5 / eta(x))^2 in powers of q.
Euler transform of period 2 sequence [ 2, -8, ...].
G.f. is a period 1 Fourier series which satisfies f(-1 / (32 t)) = 128 (t / i)^4 g(t) where q = exp(2 Pi i t) and g() is the g.f. for A227695.
G.f.: (Product_{k>0} (1 - x^(2*k))^5 / (1 - x^k))^2.
-8 * a(n) = A215600(2*n + 1).
EXAMPLE
1 + 2*x - 5*x^2 - 10*x^3 + 5*x^4 + 6*x^5 + 10*x^6 + 40*x^7 - 20*x^8 + ...
q^3 + 2*q^7 - 5*q^11 - 10*q^15 + 5*q^19 + 6*q^23 + 10*q^27 + 40*q^31 + ...
MATHEMATICA
a[ n_] := SeriesCoefficient[ (QPochhammer[ q^2]^5 / QPochhammer[ q])^2, {q, 0, n}]
PROG
(PARI) {a(n) = local(A); if( n<0, 0, A = x * O(x^n); polcoeff( (eta(x^2 + A)^5 / eta(x + A))^2, n))}
CROSSREFS
KEYWORD
sign
AUTHOR
Michael Somos, Sep 02 2013
STATUS
approved