OFFSET
0,11
COMMENTS
REFERENCES
B. C. Berndt, Ramanujan's Notebooks Part IV, Springer-Verlag, see p. 233, Entry 66.
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 q^(1/2) * (eta(q^2) / eta(q^10))^2 * eta(q^5) / eta(q) in powers of q.
Euler transform of period 10 sequence [ 1, -1, 1, -1, 0, -1, 1, -1, 1, 0, ...].
Given g.f. A(x), then B(q) = A(q^2) / q satisfies 0 = f(B(q), B(q^2)) where f(u, v) = (v^2 - u^2)^2 - (u^2 - 1) * (u^2 - 5) * v^2.
Given g.f. A(x), then B(q) = A(q^2) / q satisfies 0 = f(B(q), B(q^3)) where f(u, v) = (v^2 - u^2) * (u + v)^2 - u * v * (u^2 - 1) * (v^2 - 5).
Given g.f. A(x), then B(q) = A(q^2) / q satisfies 0 = f(B(q), B(q^2), B(q^4)) where f(u, v, w) = u^2 * w * (v^2 - 1) - v * (v + w)^2.
Given g.f. A(x), then B(q) = A(q^2) / q satisfies 0 = f(B(q), B(q^2), B(q^3), B(q^6)) where f(u1, u2, u3, u6) = (u1 * u6 - u2 * u3)^2 - u2 * u6 * (u3^2 - u1^2).
G.f. is a period 1 Fourier series which satisfies f(-1 / (10 t)) = 5^(1/2) g(t) where q = exp(2 Pi i t) and g() is the g.f. for A138526.
G.f.: (Product_{k>0} P(5,x^k) * P(10,x^k)^2)^(-1) where P(n,x) is the n-th cyclotomic polynomial.
a(5*n + 2) = a(5*n + 4) = 0.
EXAMPLE
G.f. = 1 + x + x^3 - x^5 - x^8 + 2*x^10 + x^13 - 2*x^15 - x^16 - 2*x^18 + ...
G.f. = 1/q + q + q^5 - q^9 - q^15 + 2*q^19 + q^25 - 2*q^29 - q^31 - 2*q^35 + ...
MATHEMATICA
a[ n_] := SeriesCoefficient[ x^(1/2) EllipticTheta[ 2, 0, x^(1/2)] / EllipticTheta[ 2, 0, x^(5/2)], {x, 0, n}]; (* Michael Somos, Sep 08 2015 *)
PROG
(PARI) {a(n) = my(A); if( n<0, 0, A = x * O(x^n); polcoeff( eta(x^5 + A) / eta(x + A) * (eta(x^2 + A) / eta(x^10 + A))^2, n))};
CROSSREFS
KEYWORD
sign
AUTHOR
Michael Somos, Mar 23 2008
STATUS
approved