OFFSET
-1,2
COMMENTS
LINKS
Vaclav Kotesovec, 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 q^-1 * (chi(-q) * chi(-q^11))^2 in powers of q where chi() is a Ramanujan theta function.
Expansion of (eta(q) * eta(q^11) / (eta(q^2) * eta(q^22)))^2 in powers of q.
Euler transform of period 22 sequence [ -2, 0, -2, 0, -2, 0, -2, 0, -2, 0, -4, 0, -2, 0, -2, 0, -2, 0, -2, 0, -2, 0, ...].
G.f. is a period 1 Fourier series which satisfies f(-1 / (22 t)) = 4 g(t) where q = exp(2 Pi i t) and g() is the g.f. for A123631.
G.f.: x^-1 * (Product_{k>0} (1 + x^k) * (1 + x^(11*k)))^-2.
G.f. A(x) satisfies 0 = f(A(x), A(x^2)) where f(u, v) = u^2 * v - v^2 + 4 * u + 4 * u * v.
a(n) ~ -(-1)^n * exp(2*Pi*sqrt(n/11)) / (2*11^(1/4)*n^(3/4)). - Vaclav Kotesovec, Sep 08 2017
EXAMPLE
G.f. = 1/q - 2 + q - 2*q^2 + 4*q^3 - 4*q^4 + 5*q^5 - 6*q^6 + 9*q^7 - ...
MATHEMATICA
a[ n_] := SeriesCoefficient[ (QPochhammer[ q, q^2] QPochhammer[ q^11, q^22])^2 / q, {q, 0, n}]; (* Michael Somos, Aug 27 2014 *)
PROG
(PARI) {a(n) = my(A); if( n<-1, 0, n++; A = x * O(x^n); polcoeff( (eta(x + A) * eta(x^11 + A) / (eta(x^2 + A) * eta(x^22 + A)))^2, n))};
CROSSREFS
KEYWORD
sign
AUTHOR
Michael Somos, Aug 18 2007
STATUS
approved