OFFSET
0,3
COMMENTS
LINKS
G. C. Greubel, Table of n, a(n) for n = 0..2500
Michael Somos, Introduction to Ramanujan theta functions
Eric Weisstein's World of Mathematics, Ramanujan Theta Functions
FORMULA
Expansion of q^(-3/8) * eta(q) * eta(q^4) * eta(q^6)^2 / (eta(q^2) * eta(q^12)) in powers of q.
Euler transform of period 12 sequence [0, -2, -1, -1, 0, -2, 0, -1, -1, -2, 0, -2, ...].
a(3*n) = A226860(n). a(3*n + 1) = 0.
a(3*n + 2) = -2 * A257469(n). - Michael Somos, Apr 25 2015
EXAMPLE
G.f. = 1 - 2*x^2 - x^3 + 2*x^5 + 2*x^8 - x^9 - 2*x^17 - x^18 - 2*x^20 + ...
G.f. = q^3 - 2*q^19 - q^27 + 2*q^43 + 2*q^67 - q^75 - 2*q^139 - q^147 + ...
MATHEMATICA
a[ n_] := SeriesCoefficient[ EllipticTheta[ 4, 0, q^2] EllipticTheta[ 2, Pi/4, q^(3/2)] / (Sqrt[2] q^(3/8)), {q, 0, n}];
PROG
(PARI) {a(n) = my(A); if( n<0, 0, A = x * O(x^n); polcoeff( eta(x^2 + A)^2 * eta(x^3 + A) * eta(x^12 + A) / (eta(x^4 + A) * eta(x^6 + A)), n))};
(PARI) {a(n) = my(A, p, e, i); if( n<0, 0, n = 8*n + 3; A = factor(n); -I * prod( k=1, matsize(A)[1], if( p = A[k, 1], e = A[k, 2]; if( p==2, 0, if( p==3, I^e, if( p%24 == 1 || p%24==19, for(j=1, sqrtint(p\18), if( issquare( p - 18*j^2, &i), break)); (e+1) * (if(p%24==1, 1, -I) * kronecker( 12, i))^e, if( e%2, 0, if(p%24>12, 1, -1)^(e/2)))) ))))};
CROSSREFS
KEYWORD
sign
AUTHOR
Michael Somos, Sep 08 2014
STATUS
approved