OFFSET
0,2
COMMENTS
LINKS
Seiichi Manyama, 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 phi(-x)^3 * f(-x^6)^3 / chi(-x)^2 in powers of x where phi(), chi(), f() are Ramanujan theta functions.
Expansion of q^(-5/6) * eta(q)^4 * eta(q^6)^3 / eta(q^2) in powers of q.
Euler transform of period 6 sequence [-4, -3, -4, -3, -4, -6, ...].
G.f.: Product_{k>0} (1 - x^k)^4 * (1 - x^(2*k))^2 * (1 + x^(2*k) + x^(4*k))^3.
6 * a(n) = A116418(4*n + 3).
EXAMPLE
G.f. = 1 - 4*x + 3*x^2 + 4*x^3 - x^4 - 4*x^5 - 11*x^6 + 20*x^7 - 9*x^8 + ...
G.f. = q^5 - 4*q^11 + 3*q^17 + 4*q^23 - q^29 - 4*q^35 - 11*q^41 + 20*q^47 + ...
MATHEMATICA
a[ n_] := SeriesCoefficient[ EllipticTheta[ 4, 0, x] QPochhammer[ x]^2 QPochhammer[ x^6]^3, {x, 0, n}];
a[ n_] := SeriesCoefficient[ EllipticTheta[ 4, 0, x]^3 QPochhammer[ -x, x]^2 QPochhammer[ x^6]^3, {x, 0, n}];
PROG
(PARI) {a(n) = my(A); if( n<0, 0, A = x * O(x^n); polcoeff( eta(x + A)^4 * eta(x^6 + A)^3 / eta(x^2 + A), n))};
(PARI) {a(n) = if( n<0, 0, polcoeff( prod(k=1, n, (1 - x^k)^4 * (1 - x^(2*k))^2 * (1 + x^(2*k) + x^(4*k))^3, 1 + x * O(x^n)), n))};
CROSSREFS
KEYWORD
sign
AUTHOR
Michael Somos, Jan 08 2017
STATUS
approved