OFFSET
-1,2
COMMENTS
The reason for having multiple versions of these McKay-Thompson series is that each one is a quotient of Dedekind eta-functions, differing only in their constant terms. Each one is equally important.
LINKS
G. C. Greubel, Table of n, a(n) for n = -1..1000
J. M. Borwein and P. B. Borwein, A cubic counterpart of Jacobi's identity and the AGM, Trans. Amer. Math. Soc., 323 (1991), no. 2, 691-701.
D. Ford, J. McKay and S. P. Norton, More on replicable functions, Commun. Algebra 22, No. 13, 5175-5193 (1994).
Michael Somos, Introduction to Ramanujan theta functions
Eric Weisstein's World of Mathematics, Ramanujan Theta Functions
FORMULA
Expansion of c(q) * b(q^4) / (b(q) * c(q^4)) in powers of q where b(), c() are cubic AGM theta functions.
Expansion of (1/q) * ((chi(-q^3) * chi(-q^6)) / (chi(-q) * chi(-q^3)))^4 in powers of q where chi() is a Ramanujan theta function.
Expansion of (eta(q^3) * eta(q^4) / (eta(q) * eta(q^12)))^4 in powers of q.
Euler transform of period 12 sequence [ 4, 4, 0, 0, 4, 0, 4, 0, 0, 4, 4, 0, ...].
G.f. is a period 1 Fourier series which satisfies f(-1 / (12 t)) = f(t) where q = exp(2 Pi i t).
a(n) = -(-1)^n * A193522(n). - Michael Somos, Sep 05 2015
a(n) ~ exp(2*Pi*sqrt(n/3)) / (2 * 3^(1/4) * n^(3/4)). - Vaclav Kotesovec, Sep 10 2015
EXAMPLE
G.f. = 1/q + 4 + 14*q + 36*q^2 + 85*q^3 + 180*q^4 + 360*q^5 + 684*q^6 + 1246*q^7 + ...
MATHEMATICA
a[ n_] := SeriesCoefficient[ 1/q (QPochhammer[ q^3] QPochhammer[ q^4] / (QPochhammer[ q] QPochhammer[ q^12]))^4, {q, 0, n}]; (* Michael Somos, Sep 05 2015 *)
a[ n_] := SeriesCoefficient[ 1/q (QPochhammer[ -q, q] QPochhammer[ -q^2, q^2] QPochhammer[ q^3, q^6] QPochhammer[ q^6, q^12])^4, {q, 0, n}]; (* Michael Somos, Sep 05 2015 *)
nmax = 50; CoefficientList[Series[Product[((1-x^(3*k)) * (1-x^(4*k)) / ((1-x^k) * (1-x^(12*k))))^4, {k, 1, nmax}], {x, 0, nmax}], x] (* Vaclav Kotesovec, Sep 10 2015 *)
PROG
(PARI) {a(n) = my(A); if( n<-1, 0, n++; A = x * O(x^n); polcoeff( (eta(x^3 + A) * eta(x^4 + A) / (eta(x + A) * eta(x^12 + A)))^4, n))};
CROSSREFS
KEYWORD
nonn
AUTHOR
Michael Somos, Mar 06 2011
STATUS
approved