OFFSET
-1,2
COMMENTS
LINKS
Seiichi Manyama, Table of n, a(n) for n = -1..10000
Vaclav Kotesovec, A method of finding the asymptotics of q-series based on the convolution of generating functions, arXiv:1509.08708 [math.CO], Sep 30 2015
Michael Somos, Introduction to Ramanujan theta functions
Eric Weisstein's World of Mathematics, Ramanujan Theta Functions
FORMULA
Expansion of q^(-1) * (chi(-q^7) / chi(-q))^4 in powers of q where chi() is a Ramanujan theta function.
Expansion of (eta(q^2) * eta(q^7) / (eta(q) * eta(q^14)))^4 in powers of q.
Euler transform of period 14 sequence [ 4, 0, 4, 0, 4, 0, 0, 0, 4, 0, 4, 0, 4, 0, ...].
G.f. is a period 1 Fourier series which satisfies f(-1 / (14 t)) = f(t) where q = exp(2 Pi i t).
G.f. A(x) satisfies 0 = f(A(x), A(x^2), A(x^4)) where f(u, v, w) = (u^2 - v) * (w^2 - v) - u*w * (8*(1 + v^2) - 16*v).
G.f. A(x) satisfies 0 = f(A(x), A(x^2)) where f(u, v) = (u*v + 1) * (u + v) * (u - v)^2 - u*v * (u - 1) * (v - 1) * (u*v - 8*(u+v) + 1).
G.f.: (1/x) * (Product_{k>0} P(x^k))^-4 where P(x) = x^6 - x^5 + x^4 - x^3 + x^2 - x + 1 is the 14th cyclotomic polynomial.
a(n) = A058504(n) unless n = 0.
a(n) ~ exp(2*Pi*sqrt(2*n/7)) / (2^(3/4) * 7^(1/4) * n^(3/4)). - Vaclav Kotesovec, Oct 14 2015
EXAMPLE
G.f. = 1/q + 4 + 10*q + 24*q^2 + 51*q^3 + 100*q^4 + 190*q^5 + 340*q^6 + ...
MATHEMATICA
a[ n_] := SeriesCoefficient[ 1/q (QPochhammer[ -q, q] / QPochhammer[ -q^7, q^7])^4, {q, 0, n}]; (* Michael Somos, Apr 26 2015 *)
a[ n_] := SeriesCoefficient[ 1/q (QPochhammer[ q^2] QPochhammer[ q^7] / (QPochhammer[ q] QPochhammer[ q^14]))^4, {q, 0, n}]; (* Michael Somos, Apr 26 2015 *)
nmax=60; CoefficientList[Series[Product[((1+x^k) / (1+x^(7*k)))^4, {k, 1, nmax}], {x, 0, nmax}], x] (* Vaclav Kotesovec, Oct 14 2015 *)
PROG
(PARI) {a(n) = my(A); if( n<-1, 0, n++; A = x * O(x^n); polcoeff( (eta(x^2 + A) * eta(x^7 + A) / (eta(x + A) * eta(x^14 + A)))^4, n))};
CROSSREFS
KEYWORD
nonn
AUTHOR
Michael Somos, Mar 06 2007
STATUS
approved