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 f(-x^3)^3 / (f(x, x^2) * psi(x)) in powers of x where psi(), f(, ) are Ramanujan theta functions.
Expansion of q^(-5/24) * eta(q)^2 * eta(q^3) * eta(q^6) / eta(q^2)^3 in powers of q.
Euler transform of period 6 sequence [ -2, 1, -3, 1, -2, -1, ...].
a(n) = A053269(3*n + 1).
a(n) ~ (-1)^n * exp(sqrt(n/2)*Pi) / (6*sqrt(n)). - Vaclav Kotesovec, Apr 17 2016
EXAMPLE
G.f. = 1 - 2*x + 2*x^2 - 5*x^3 + 9*x^4 - 12*x^5 + 16*x^6 - 23*x^7 +
G.f. = q^5 - 2*q^29 + 2*q^53 - 5*q^77 + 9*q^101 - 12*q^125 + 16*q^149 - 23*q^173 + ...
MATHEMATICA
a[ n_] := SeriesCoefficient[ 2 x^(1/8) QPochhammer[ x^3]^3 QPochhammer[ x, x^2] / (EllipticTheta[ 4, 0, x^3] EllipticTheta[ 2, 0, x^(1/2)]), {x, 0, n}];
PROG
(PARI) {a(n) = my(A); if( n<0, 0, A = x * O(x^n); polcoeff( eta(x + A)^2 * eta(x^3 + A) * eta(x^6 + A) / eta(x^2 + A)^3, n))};
CROSSREFS
KEYWORD
sign
AUTHOR
Michael Somos, Apr 17 2016
STATUS
approved