OFFSET
0,2
REFERENCES
J. H. Conway and N. J. A. Sloane, "Sphere Packings, Lattices and Groups", Springer-Verlag, p. 118.
LINKS
Seiichi Manyama, Table of n, a(n) for n = 0..10000
FORMULA
G.f.: (theta_3(q^(1/2))^7 + theta_4(q^(1/2))^7)/2.
EXAMPLE
1 + 84*q^2 + 574*q^4 + 1288*q^6 + 3444*q^8 + ...
MATHEMATICA
terms = 34; s = 1/2 (EllipticTheta[3, 0, q]^7 + EllipticTheta[4, 0, q]^7) + O[q]^(2 terms); CoefficientList[s, q^2] (* Jean-François Alcover, Jul 07 2017 *)
PROG
(PARI) {a(n)=if(n<0, 0, n*=2; polcoeff( sum(k=1, sqrtint(n), 2*x^k^2, 1+x*O(x^n))^7, n))} /* Michael Somos, Nov 03 2006 */
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
STATUS
approved