OFFSET
0,3
COMMENTS
REFERENCES
B. C. Berndt, Ramanujan's Notebooks Part III, Springer-Verlag, see p. 226 Entry 4(ii).
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
Euler transform of period 12 sequence [1, -6, 6, -5, 1, -12, 1, -5, 6, -6, 1, -6, ...].
Expansion of eta(q^2)^7 * eta(q^6)^11 / (eta(q) * eta(q^3)^5 * eta(q^4) * eta(q^12)^5) in powers of q.
a(n) is multiplicative and a(3^e) = 1, a(2^e) = ((-4)^(e+1)-1)/(-4-1)-2, a(p^e) = ((-p^2)^(e+1)-1)/(-p-1) if p == 2 (mod 3), a(p^e) = ((p^2)(e+1)-1)/(p-1) if p == 1 (mod 3).
G.f.: 1 + Sum_{k>0} k^2 x^k/(1-(-x)^k) Kronecker(-3, k).
EXAMPLE
G.f. = 1 + q - 5*q^2 + q^3 + 11*q^4 - 24*q^5 - 5*q^6 + 50*q^7 + ... - Michael Somos, Sep 07 2018
MATHEMATICA
a[n_]:= SeriesCoefficient[(9*EllipticTheta[3, 0, q]*EllipticTheta[3, 0, q^3]^5 - EllipticTheta[3, 0, q]^5*EllipticTheta[3, 0, q^3])/8, {q, 0, n}]; Table[a[n], {n, 0, 50}] (* G. C. Greubel, Feb 09 2018 *)
PROG
(PARI) {a(n) = if( n<1, n==0, sumdiv(n, d, d^2 * kronecker(-3, d) * (-1)^(n-d)))};
(PARI) {a(n) = my(A); if( n<0, 0, A = x * O(x^n); polcoeff( eta(x^2 + A)^7 * eta(x^6 + A)^11 / (eta(x + A) * eta(x^3 + A)^5 * eta(x^4 + A) * eta(x^12 + A)^5), n))};
(PARI) {a(n) = my(A, p, e, t); if( n<1, n==0, A = factor(n); prod(k=1, matsize(A)[1], [p, e] = A[k, ]; if( p==3, 1, t=p^2*(-1)^(p%3==2); (t^(e+1) - 1) / (t-1) - 2*(p==2))))};
CROSSREFS
KEYWORD
sign,mult
AUTHOR
Michael Somos, Oct 21 2005
STATUS
approved