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

Terms

a(0) =1a(1) =2a(2) =3a(3) =5a(4) =6a(5) =9a(6) =9a(7) =10a(8) =13a(9) =15a(10) =17a(11) =16a(12) =17a(13) =23a(14) =22a(15) =25a(16) =23a(17) =27a(18) =28a(19) =31a(20) =33a(21) =30a(22) =37a(23) =35a(24) =36a(25) =43a(26) =41a(27) =43a(28) =42a(29) =45