OFFSET
0,2
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 q^(-1/2) *( (eta(q)*eta(q^3))^3 + 9*(eta(q^3)*eta(q^9))^3 ) in powers of q.
a(n) = b(2*n + 1) where b() is multiplicative with b(2^e) = 0^e, b(3^e) = -2 * (-3)^e if e>0, b(p^e) = p^e * (1 + (-1)^e) / 2 if p == 5 (mod 6), b(p^e) = b(p) * b(p^(e-1)) - p^2 * b(p^(e-2)) if p == 1 (mod 6).
G.f. is a period 1 Fourier series which satisfies f(-1 / (36 t)) = 139968^(1/2) (t/i)^3 g(t) where q = exp(2 Pi i t) and g() is the g.f. for A152243.
EXAMPLE
G.f. = 1 + 6*x + 2*x^3 - 18*x^4 - 22*x^6 + 26*x^9 + 12*x^10 + 25*x^11 + ...
G.f. = q + 6*q^3 + 2*q^7 - 18*q^9 - 22*q^13 + 26*q^19 + 12*q^21 + 25*q^25 + ...
MATHEMATICA
a[ n_] := SeriesCoefficient[ (QPochhammer[ x] QPochhammer[ x^3])^3 + 9 x (QPochhammer[ x^3] QPochhammer[ x^9])^3 , {x, 0, n}]; (* Michael Somos, Sep 02 2015 *)
PROG
(PARI) {a(n) = my(A); if( n<0, 0, A = x * O(x^n); polcoeff( (eta(x + A) * eta(x^3 + A))^3 + 9 * x * (eta(x^3 + A) * eta(x^9 + A))^3, n))};
CROSSREFS
KEYWORD
sign
AUTHOR
Michael Somos, Nov 30 2008
STATUS
approved