OFFSET
-1,2
COMMENTS
LINKS
G. C. Greubel, Table of n, a(n) for n = -1..1000
Michael Somos, Introduction to Ramanujan theta functions
Eric Weisstein's World of Mathematics, Ramanujan Theta Functions
FORMULA
Expansion of 3 + psi(-q) / (q * psi(-q^9)) + 3 * q * psi(-q^9) / psi(-q) in powers of q where psi() is a Ramanujan theta function.
Expansion of (1/q) * (psi(-q^3)^2 / (psi(-q) * psi(-q^9)))^2 in powers of q where psi() is a Ramanujan theta function.
Expansion of -3 * b(-q) * c(-q) * (b(q^9) / (b(q^2) * c(q^2) * b(-q^3)))^2 in powers of q where b(), c() are cubic AGM theta functions.
Euler transform of period 36 sequence [ 2, 0, -2, 2, 2, 0, 2, 2, 0, 0, 2, -2, 2, 0, -2, 2, 2, 0, 2, 2, -2, 0, 2, -2, 2, 0, 0, 2, 2, 0, 2, 2, -2, 0, 2, 0, ...].
G.f. is a period 1 Fourier series which satisfies f(-1 / (36 t)) = f(t) where q = exp(2 Pi i t).
Convolution square of A112205.
a(n) ~ exp(2*Pi*sqrt(n)/3) / (2*sqrt(3)*n^(3/4)). - Vaclav Kotesovec, Nov 12 2015
EXAMPLE
1/q + 2 + 3*q + 2*q^2 + 3*q^3 + 6*q^4 + 10*q^5 + 12*q^6 + 15*q^7 + 22*q^8 + ...
MATHEMATICA
nmax = 60; CoefficientList[Series[Product[((1+x^k) * (1-x^(3*k))^2 * (1+x^(6*k))^2 * (1+x^(9*k)) / ((1-x^(4*k)) * (1-x^(36*k))))^2, {k, 1, nmax}], {x, 0, nmax}], x] (* Vaclav Kotesovec, Nov 12 2015 *)
a[n_]:= SeriesCoefficient[3 - EllipticTheta[2, 0, I*q^(1/2)]/EllipticTheta[2, 0, I*q^(9/2)] - 3*EllipticTheta[2, 0, I*q^(9/2)]/EllipticTheta[2, 0, q^(1/2)], {q, 0, n}]; Table[a[n], {n, -1, 50}] (* G. C. Greubel, Feb 18 2018 *)
PROG
(PARI) {a(n) = local(A); if( n<-1, 0, n++; A = x * O(x^n); polcoeff( (eta(x^2 + A) * eta(x^3 + A)^2 * eta(x^12 + A)^2 * eta(x^18 + A) / (eta(x + A) * eta(x^4 + A) * eta(x^6 + A)^2 * eta(x^9 + A) * eta(x^36 + A)))^2, n))}
CROSSREFS
KEYWORD
nonn
AUTHOR
Michael Somos, Jul 16 2013
STATUS
approved