OFFSET
0,2
REFERENCES
J. H. Conway and N. J. A. Sloane, "Sphere Packings, Lattices and Groups", Springer-Verlag, p. 123.
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
LINKS
T. D. Noe, Table of n, a(n) for n = 0..1000
N. Heninger, E. M. Rains and N. J. A. Sloane, On the Integrality of n-th Roots of Generating Functions, J. Combinatorial Theory, Series A, 113 (2006), 1732-1745.
G. Nebe and N. J. A. Sloane, Home page for this lattice
FORMULA
Expansion of eta(q)^9 / eta(q^3)^3 + 81*q * eta(q^3)^9 / eta(q)^3 in powers of q.
Expansion of a(q)^3 + 2*c(q)^3 in powers of q where a(), c() are cubic AGM theta functions. - Michael Somos, Oct 24 2006
MATHEMATICA
a[ n_] := SeriesCoefficient[ QPochhammer[ q]^9 / QPochhammer[ q^3]^3 + 81 q QPochhammer[ q^3]^9 / QPochhammer[ q]^3, {q, 0, n}]; (* Michael Somos, Feb 19 2015 *)
terms = 37; f[q_] = LatticeData["E6", "ThetaSeriesFunction"][-I Log[q]/Pi]; s = Series[f[q], {q, 0, 2 terms}] // Normal // Simplify[#, q > 0]&; (List @@ s)[[1 ;; terms]] /. q -> 1 (* Jean-François Alcover, Jul 04 2017 *)
PROG
(PARI) {a(n) = local(A); if( n<0, 0, A = x * O(x^n); polcoeff( eta(x + A)^9 / eta(x^3 + A)^3 + 81 * x * eta(x^3 + A)^9 / eta(x + A)^3, n))}; /* Michael Somos, Oct 24 2006 */
CROSSREFS
KEYWORD
nonn
AUTHOR
STATUS
approved