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 (phi(q) * phi(q^2) * phi(-q)^2)^2 in powers of q where phi() is a Ramanujan theta function.
Euler transform of period 8 sequence [ -4, -6, -4, -12, -4, -6, -4, -8, ...].
G.f.: Product_{k>0} ((1 - x^k)^4 * (1 + x^k)^2 * (1 + x^(2*k)) / (1 + x^(4*k))^2)^2.
Convolution square of A131999.
EXAMPLE
G.f. = 1 - 4*q + 16*q^3 - 16*q^4 + 8*q^5 - 96*q^7 + 112*q^8 + 44*q^9 + ...
MATHEMATICA
a[ n_] := SeriesCoefficient[ (QPochhammer[ q]^2 QPochhammer[ q^2] QPochhammer[ q^4]^3 / QPochhammer[ q^8]^2)^2, {q, 0, n}];
a[ n_] := SeriesCoefficient[ (EllipticTheta[ 3, 0, q] EllipticTheta[ 3, 0, q^2] EllipticTheta[ 4, 0, q]^2)^2, {q, 0, n}];
a[ n_] := SeriesCoefficient[ (EllipticTheta[ 4, 0, q] EllipticTheta[ 4, 0, q^2] EllipticTheta[ 4, 0, q^4]^2)^2, {q, 0, n}];
PROG
(PARI) {a(n) = my(A); if( n<0, 0, A = x * O(x^n); polcoeff( (eta(x + A)^2 * eta(x^2 + A) * eta(x^4 + A)^3 / eta(x^8 + A)^2)^2, n))};
(Magma) A := Basis( ModularForms( Gamma1(8), 4), 52); A[1] - 4*A[2] + 16*A[4] - 16*A[5] + 8*A[6] - 96*A[8] + 112*A[9];
CROSSREFS
KEYWORD
sign
AUTHOR
Michael Somos, Jun 28 2015
STATUS
approved