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, Comm. Algebra 22, No. 13, 5175-5193 (1994).
Michael Somos, Introduction to Ramanujan theta functions
Eric Weisstein's World of Mathematics, Ramanujan Theta Functions
FORMULA
Expansion of (1/q) * (phi(q) * psi(q)) / (psi(q^3) * psi(q^6)) in powers of q where phi(), psi() are Ramanujan theta functions.
Expansion of eta(q^2)^7 * eta(q^3) / (eta(q)^3 * eta(q^4)^2 * eta(q^6) * eta(q^12)^2) in powers of q.
Euler transform of period 12 sequence [ 3, -4, 2, -2, 3, -4, 3, -2, 2, -4, 3, 0, ...].
a(2*n) = 0 unless n=0. a(2*n - 1) = A058487(n).
EXAMPLE
G.f. = 1/q + 3 + 2*q + q^3 - 2*q^7 - 2*q^9 + 2*q^11 + 4*q^13 + 3*q^15 - 4*q^17 + ...
MATHEMATICA
a[ n_] := SeriesCoefficient[ 2 EllipticTheta[ 3, 0, q] EllipticTheta[ 2, 0, q^(1/2)]/(EllipticTheta[ 2, 0, q^(3/2)] EllipticTheta[ 2, 0, q^3]), {q, 0, n}]; (* Michael Somos, Sep 05 2015 *)
PROG
(PARI) {a(n) = my(A); if( n<-1, 0, n++; A = x * O(x^n); polcoeff( eta(x^2 + A)^7 * eta(x^3 + A) / (eta(x + A)^3 * eta(x^4 + A)^2 * eta(x^6 + A) * eta(x^12 + A)^2), n))};
CROSSREFS
KEYWORD
sign
AUTHOR
Michael Somos, Mar 05 2011
STATUS
approved