a(n) is the number of Gaussian integers z=a+bi satisfying |z|<=n, a>0, b>=0.

Terms

a(0) =0a(1) =1a(2) =3a(3) =7a(4) =12a(5) =20a(6) =28a(7) =37a(8) =49a(9) =63a(10) =79a(11) =94a(12) =110a(13) =132a(14) =153a(15) =177a(16) =199a(17) =225a(18) =252a(19) =282a(20) =314a(21) =343a(22) =379a(23) =413a(24) =448a(25) =490a(26) =530a(27) =572a(28) =613a(29) =657