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 q^-1 * (chi(q) * chi(q^5))^4 in powers of q where chi() is a Ramanujan theta function.
Euler transform of period 20 sequence [ 4, -4, 4, 0, 8, -4, 4, 0, 4, -8, 4, 0, 4, -4, 8, 0, 4, -4, 4, 0, ...].
G.f. is a period 1 Fourier series which satisfies f(-1 / (20 t)) = f(t) where q = exp(2 Pi i t).
G.f.: (Product_{k>0} (1 + (-x)^k) * (1 + (-x)^(5*k)))^-4.
a(n) = A112158(n) unless n=0. a(n) = 5*A210458(n) + A145740(n) unless n=0. A132040(n) = (-1)^n * a(n). Convolution square of A112179.
a(n) ~ exp(2*Pi*sqrt(n/5)) / (2 * 5^(1/4) * n^(3/4)). - Vaclav Kotesovec, Apr 30 2017
EXAMPLE
G.f. = 1/q + 4 + 6*q + 8*q^2 + 17*q^3 + 32*q^4 + 54*q^5 + 80*q^6 + 116*q^7 + ...
MATHEMATICA
a[ n_] := SeriesCoefficient[ 1/q (QPochhammer[ -q, q^2] QPochhammer[ -q^5, q^10])^4, {q, 0, n}]; (* Michael Somos, Aug 26 2015 *)
nmax = 50; CoefficientList[Series[Product[((1 + x^(2*k-1))/((1 + x^(10*k))*(1 - x^(10*k-5))))^4, {k, 1, nmax}], {x, 0, nmax}], x] (* Vaclav Kotesovec, Apr 30 2017 *)
PROG
(PARI) {a(n) = my(A); if( n<-1, 0, n++; A = x * O(x^n); polcoeff( (eta(x^2 + A)^2 * eta(x^10 + A)^2 / (eta(x + A) * eta(x^4 + A) * eta(x^5 + A) * eta(x^20 + A)))^4, n))};
CROSSREFS
KEYWORD
nonn
AUTHOR
Michael Somos, Jan 21 2013
STATUS
approved