OFFSET
0,3
FORMULA
a(n) = 1/4 * Sum_{i,j in Z and i^2 + j^2 <= n} i^2 + j^2.
G.f.: (1/(1-x)) * Sum_{k>=1} k * x^k * (1-x^(2*k)) / (1+x^(2*k))^2.
PROG
(PARI) my(N=60, x='x+O('x^N)); concat(0, Vec(sum(k=1, N, k*x^k*(1-x^(2*k))/(1+x^(2*k))^2)/(1-x)))
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, Jul 11 2024
STATUS
approved