OFFSET
1,3
COMMENTS
LINKS
G. C. Greubel, Table of n, a(n) for n = 1..5000
Michael Somos, Introduction to Ramanujan theta functions
Eric Weisstein's World of Mathematics, Ramanujan Theta Functions
FORMULA
Expansion of eta(q^2)^4 * eta(q^7) * eta(q^28) / (eta(q) * eta(q^4) * eta(q^14)) in powers of q.
Euler transform of period 28 sequence [ 1, -3, 1, -2, 1, -3, 0, -2, 1, -3, 1, -2, 1, -3, 1, -2, 1, -3, 1, -2, 0, -3, 1, -2, 1, -3, 1, -3, ...].
G.f. is a period 1 Fourier series which satisfies f(-1 / (28 t)) = 392^(1/2) (t/i)^(3/2) g(t) where q = exp(2 Pi i t) and g() is the g.f. for A159813.
a(7*n) = A159813(n).
EXAMPLE
G.f. = q + q^2 - 2*q^3 - q^4 - 2*q^6 + q^7 - q^8 - q^9 + 4*q^10 + 2*q^11 + 2*q^12 + ...
MATHEMATICA
a[ n_] := SeriesCoefficient[ EllipticTheta[ 3, 0, q] EllipticTheta[ 2, Pi/4, q^(1/2)] EllipticTheta[ 2, Pi/4, q^(7/2)] / 2, {q, 0, n}]; (* Michael Somos, Aug 27 2015 *)
PROG
(PARI) {a(n) = my(A); if( n<1, 0, n--; A = x * O(x^n); polcoeff( eta(x^2 + A)^4 * eta(x^7 + A) * eta(x^28 + A) / (eta(x + A) * eta(x^4 + A) * eta(x^14 + A)), n))};
CROSSREFS
KEYWORD
sign
AUTHOR
Michael Somos, Aug 15 2012
STATUS
approved