OFFSET
-1,2
COMMENTS
LINKS
G. C. Greubel, Table of n, a(n) for n = -1..1000
D. Ford, J. McKay and S. P. Norton, More on replicable functions, Commun. Algebra 22, No. 13, 5175-5193 (1994). See Table 4 18C.
Michael Somos, Introduction to Ramanujan theta functions
Eric Weisstein's World of Mathematics, Ramanujan Theta Functions
FORMULA
Expansion of -4 + psi(q) / (q * psi(q^9)) + 3 * q * psi(q^9) / psi(q) in powers of q where psi() is a Ramanujan theta function. - Michael Somos, Aug 09 2012
Expansion of (1/q) * (chi(-q) * chi(-q^9))^3 / chi(-q^3)^2 in powers of q where chi() is a Ramanujan theta function.
Expansion of b(q) * c(q^3) / (b(q^2) * c(q^6)) in powers of q where b(), c() are cubic AGM theta functions.
Given g.f. A(x), then B(x) = 1/A(x) satisfies 0 = f(B(x), B(x^2)) where f(u, v) = u^2 - v - u * v * (6 + 4*v).
G.f. is a period 1 Fourier series which satisfies f(-1 / (18 t)) = 4 * g(t) where q = exp(2 Pi i t) and g() is g.f. for A123629.
Convolution inverse of A123629.
a(n) ~ (-1)^(n+1) * exp(2*Pi*sqrt(n)/3) / (2*sqrt(3)*n^(3/4)). - Vaclav Kotesovec, Nov 12 2015
EXAMPLE
1/q - 3 + 3*q - 2*q^2 + 3*q^3 - 6*q^4 + 10*q^5 - 12*q^6 + 15*q^7 - 22*q^8 + ...
MATHEMATICA
QP = QPochhammer; s = (QP[q]*(QP[q^9]/(QP[q^2]*QP[q^18])))^3*(QP[q^6]/ QP[q^3])^2 + O[q]^50; CoefficientList[s, q] (* Jean-François Alcover, Nov 12 2015, adapted from PARI *)
nmax = 60; CoefficientList[Series[Product[(1+x^(3*k))^2 / ( (1+x^k)^3 * (1+x^(9*k))^3), {k, 1, nmax}], {x, 0, nmax}], x] (* Vaclav Kotesovec, Nov 12 2015 *)
PROG
(PARI) {a(n) = local(A); if( n<-1, 0, n++; A = x*O(x^n); polcoeff( (eta(x + A) * eta(x^9 + A) / (eta(x^2 + A) * eta(x^18 + A)))^3 * (eta(x^6 + A) / eta(x^3 + A))^2, n))}
CROSSREFS
KEYWORD
sign
AUTHOR
Michael Somos, Oct 05 2006
STATUS
approved