OFFSET
0,11
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 1 + eta(q^3) * eta(q^18)^3 / (eta(q^6) * eta(q^9)^3) in powers of q.
Expansion of 1 + q * chi(-q^3) / chi(-q^9)^3 = psi(q) * chi(-q^3) / phi(-q^9) in powers of q where phi(), psi(), chi() are Ramanujan theta functions.
Expansion of eta(q^2)^2 * eta(q^3) * eta(q^18) / (eta(q) * eta(q^6) * eta(q^9)^2) in powers of q.
Euler transform of period 18 sequence [ 1, -1, 0, -1, 1, -1, 1, -1, 2, -1, 1, -1, 1, -1, 0, -1, 1, 0, ...].
G.f. is a period 1 Fourier series which satisfies f(-1 / (18 t)) = (3/2) * g(t) where q = exp(2 Pi i t) and g() is g.f. for A145977.
G.f.: Product_{k>0} P(2, x^k)^2 * P(18, x^k) / (P(3, x^k) * P(9, x^k)) where P(n, x) is n-th cyclotomic polynomial.
EXAMPLE
1 + q - q^4 + 2*q^10 - 2*q^13 - q^16 + 4*q^19 - 4*q^22 - q^25 + 8*q^28 + ...
MATHEMATICA
eta[x_] := QPochhammer[x]; A139032[n_] := SeriesCoefficient[eta[q^2]^2 *eta[q^3]*eta[q^18]/(eta[q]*eta[q^6]*eta[q^9]^2), {q, 0, n}];
Table[A139032[n], {n, 0, 20}] (* G. C. Greubel, Aug 09 2017 *)
PROG
(PARI) {a(n) = local(A); if( n<0, 0, A = x * O(x^n); polcoeff( eta(x^2 + A)^2 * eta(x^3 + A) * eta(x^18 + A) / (eta(x + A) * eta(x^6 + A) * eta(x^9 + A)^2), n))}
CROSSREFS
KEYWORD
sign
AUTHOR
Michael Somos, Apr 07 2008
STATUS
approved