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

Terms

a(0) =1a(1) =4a(2) =9a(3) =18a(4) =29a(5) =46a(6) =63a(7) =82a(8) =107a(9) =136a(10) =169a(11) =200a(12) =233a(13) =278a(14) =321a(15) =370a(16) =415a(17) =468a(18) =523a(19) =584a(20) =649a(21) =708a(22) =781a(23) =850a(24) =921a(25) =1006a(26) =1087a(27) =1172a(28) =1255a(29) =1344