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

Terms

a(0) =1a(1) =2a(2) =4a(3) =7a(4) =10a(5) =15a(6) =20a(7) =25a(8) =32a(9) =40a(10) =49a(11) =57a(12) =66a(13) =78a(14) =89a(15) =102a(16) =114a(17) =128a(18) =142a(19) =158a(20) =175a(21) =190a(22) =209a(23) =227a(24) =245a(25) =267a(26) =288a(27) =310a(28) =331a(29) =354