OFFSET
-1,9
COMMENTS
LINKS
G. C. Greubel, Table of n, a(n) for n = -1..2500
Michael Somos, Introduction to Ramanujan theta functions
Eric Weisstein's World of Mathematics, Ramanujan Theta Functions
FORMULA
Expansion of q^(-1) * chi(q) * chi(-q^2) * chi(q^7) * chi(-q^14) in power of q where chi() is a Ramanujan theta function.
Expansion of (eta(q^2) * eta(q^14))^3 / (eta(q) * eta(q^4)^2 * eta(q^7) * eta(q^28)^2) in powers of q.
Euler transform of period 28 sequence [ 1, -2, 1, 0, 1, -2, 2, 0, 1, -2, 1, 0, 1, -4, 1, 0, 1, -2, 1, 0, 2, -2, 1, 0, 1, -2, 1, 0, ...].
G.f. A(x) satisfies 0 = f(A(x), A(x^2)) where f(u, v) = (v - 2)^2 - u * v * (u - 2).
G.f. is a period 1 Fourier series which satisfies f(-1 / (28 t)) = 2 g(t) where q = exp(2 Pi i t) and g() is the g.f. of A123862.
EXAMPLE
G.f. = 1/q + 1 - q + q^3 - q^5 + 3*q^7 - 2*q^9 + 2*q^11 - 5*q^13 + ...
MATHEMATICA
a[ n_] := SeriesCoefficient[ q^-1 QPochhammer[ -q] QPochhammer[ -q^7] / (QPochhammer[ q^4] QPochhammer[ q^28]), {q, 0, n}]
PROG
(PARI) {a(n) = local(A); if( n<-1, 0, n++; A = x * O(x^n); polcoeff( (eta(x^2 + A) * eta(x^14 + A))^3 / (eta(x + A) * eta(x^4 + A)^2 * eta(x^7 + A) * eta(x^28 + A)^2), n))}
CROSSREFS
KEYWORD
sign
AUTHOR
Michael Somos, Oct 18 2013
STATUS
approved