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^(-1/3) * (eta(q) * eta(q^3) * eta(q^6) / eta(q^2)^3)^2 in powers of q.
Euler transform of period 6 sequence [ -2, 4, -4, 4, -2, 0, ...].
G.f.: Product_{k>0} (1 - x^k + x^(2*k))^2 * (1 + x^k + x^(2*k))^4 / (1 + x^k)^4.
a(n) = A258100(3*n + 1).
a(n) ~ (-1)^n * exp(2*Pi*sqrt(n/3)) / (2 * 3^(9/4) * n^(3/4)). - Vaclav Kotesovec, Jun 06 2018
EXAMPLE
G.f. = 1 - 2*x + 5*x^2 - 12*x^3 + 26*x^4 - 50*x^5 + 92*x^6 - 168*x^7 + ...
G.f. = q - 2*q^4 + 5*q^7 - 12*q^10 + 26*q^13 - 50*q^16 + 92*q^19 + ...
MATHEMATICA
a[ n_] := SeriesCoefficient[ (QPochhammer[ x] * QPochhammer[ x^3] * QPochhammer[ x^6] / QPochhammer[ x^2]^3)^2, {x, 0, n}];
PROG
(PARI) {a(n) = my(A); if( n<0, 0, A = x * O(x^n); polcoeff( (eta(x + A) * eta(x^3 + A) * eta(x^6 + A) / eta(x^2 + A)^3)^2, n))};
CROSSREFS
KEYWORD
sign
AUTHOR
Michael Somos, May 20 2015
STATUS
approved