OFFSET
0,7
LINKS
Michael Somos, Introduction to Ramanujan theta functions
Eric Weisstein's World of Mathematics, Ramanujan Theta Functions
FORMULA
Expansion of q^(1/6) * eta(q) / (eta(q^2) * eta(q^3)) in powers of q.
Euler transform of period 6 sequence [ -1, 0, 0, 0, -1, 1, ...].
G.f. is a period 1 Fourier series which satisfies f(-1 / (864 t)) = 24^(-1/2) (t/i)^(-1/2) g(t) where q = exp(2 Pi i t) and g() is the g.f. for A007690.
G.f.: (Product_{k>0} (1 + x^k) * (1 - x^(3*k)))^(-1).
EXAMPLE
G.f. = 1 - x - x^5 + 2*x^6 - 2*x^7 + x^8 - 2*x^11 + 5*x^12 - 5*x^13 + 2*x^14 + ...
G.f. = 1/q - q^5 - q^29 + 2*q^35 - 2*q^41 + q^47 - 2*q^65 + 5*q^71 - 5*q^77 + ...
MATHEMATICA
a[ n_] := SeriesCoefficient[ QPochhammer[ x] / (QPochhammer[ x^2] QPochhammer[ x^3]), {x, 0, n}]; (* Michael Somos, Oct 12 2015 *)
PROG
(PARI) {a(n) = my(A); if( n<0, 0, A =x * O(x^n); polcoeff( eta(x + A) / eta(x^2 + A) / eta(x^3 + A), n))};
CROSSREFS
KEYWORD
sign
AUTHOR
Michael Somos, Jan 26 2008
STATUS
approved