a(n)=number of Gaussian integers z=a+bi satisfying |z|<=n+1/2, a>0, b>=0.

Terms

a(0) =0a(1) =2a(2) =5a(3) =9a(4) =17a(5) =24a(6) =34a(7) =44a(8) =56a(9) =73a(10) =87a(11) =105a(12) =122a(13) =144a(14) =166a(15) =187a(16) =215a(17) =243a(18) =271a(19) =300a(20) =328a(21) =364a(22) =399a(23) =435a(24) =471a(25) =513a(26) =554a(27) =594a(28) =640a(29) =683