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^3) * a(-q) in powers of q where a() is a cubic AGM theta function.
G.f. is a period 1 Fourier series which satisfies f(-1 / (12 t)) = -12 * (t/i)^2 f(t) where q = exp(2 Pi i t).
a(n) = -4 * b(n) where b(n) is multiplicative and b(2^e) = 3 if e>0, b(3^e) = 1, b(p^e) = (p^(e+1) - 1)/(p - 1), if p>3. - Michael Somos, Nov 19 2013
G.f.: 1 - 4 * ( 1*x^1 / (1 + (-x)^1) + 2*x^2 / (1 + (-x)^2) + 4*x^4 / (1 + (-x)^4) + 5*x^5 / (1 + (-x)^5) + 7*x^7 / (1 + (-x)^7) + ...).
a(2*n) = a(4*n) = A186100(n). a(3*n) = a(n).
EXAMPLE
G.f. = 1 - 4*q - 12*q^2 - 4*q^3 - 12*q^4 - 24*q^5 - 12*q^6 - 32*q^7 + ...
MATHEMATICA
a[ n_]:= SeriesCoefficient[(3*EllipticTheta[3, 0, q^3]^4 - EllipticTheta[ 3, 0, q]^4)/2, {q, 0, n}];
PROG
(PARI) {a(n) = if( n<1, n==0, -4 * sumdiv( n, d, d * (2 + (-1)^(n/d)) * kronecker( 36, d)))}
(PARI) {a(n) = local(A); if( n<0, 0, A = x * O(x^n); polcoeff( (3 * sum( k=1, sqrtint(n\3), 2 * x^(3*k^2), 1 + A)^4 - sum( k=1, sqrtint(n), 2 * x^k^2, 1 + A)^4) / 2, n))}
(PARI) {a(n) = local(A, p, e); if( n<1, n==0, A = factor(n); -4 * prod( k=1, matsize(A)[1], if( p=A[k, 1], e=A[k, 2]; if( p==2, 3, if( p==3, 1, (p^(e+1) - 1)/(p - 1))))))} /* Michael Somos, Nov 19 2013 */
CROSSREFS
KEYWORD
sign
AUTHOR
Michael Somos, Oct 07 2013
STATUS
approved