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).
Eric Weisstein's World of Mathematics, Ramanujan Theta Functions
FORMULA
Expansion of (1/q) * (chi(-q) * chi(-q^2) * chi(-q^3) * chi(-q^6))^2 in powers of q where chi() is a Ramanujan theta function.
Expansion of (eta(q) * eta(q^3) / (eta(q^4) * eta(q^12)))^2 in powers of q.
Euler transform of period 12 sequence [ -2, -2, -4, 0, -2, -4, -2, 0, -4, -2, -2, 0, ...].
G.f. is a period 1 Fourier series which satisfies f(-1 / (12 t)) = 16 g(t) where q = exp(2 Pi i t) and g() is the g.f. for A123647.
Convolution inverse of A123647. - Michael Somos, Sep 02 2015
G.f. A(x) satisfies 0 = f(A(x), A(x^2)) where f(u, v) = v^2 - u * (u + 4) * (v + 4). - Michael Somos, Sep 02 2015
EXAMPLE
G.f. = 1/q - 2 - q + 7*q^3 - 9*q^5 + 10*q^7 - 23*q^9 + 38*q^11 - 47*q^13 + ...
MATHEMATICA
a[ n_] := SeriesCoefficient[ (1/q) (QPochhammer[ q] QPochhammer[ q^3] / (QPochhammer[ q^4] QPochhammer[ q^12]))^2, {q, 0, n}]; (* Michael Somos, Sep 02 2015 *)
PROG
(PARI) {a(n) = my(A); if( n<-1, 0, n++; A = x * O(x^n); polcoeff( (eta(x + A) * eta(x^3 + A) / (eta(x^4 + A) * eta(x^12 + A)))^2, n))};
CROSSREFS
KEYWORD
sign
AUTHOR
Michael Somos, Mar 06 2011
STATUS
approved