OFFSET
0,3
FORMULA
a(n) = 1/8 * Sum_{i,j,k,l in Z and i^2 + j^2 + k^2 + l^2 <= n} i^2 + j^2 + k^2 + l^2.
G.f.: (1/(1-x)) * Sum_{k>=1} k^2 * x^k / (1+(-x)^k)^2.
PROG
(PARI) my(N=50, x='x+O('x^N)); concat(0, Vec(sum(k=1, N, k^2*x^k/(1+(-x)^k)^2)/(1-x)))
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, Jul 11 2024
STATUS
approved