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

Terms

a(0) =1a(1) =9a(2) =21a(3) =37a(4) =69a(5) =97a(6) =137a(7) =177a(8) =225a(9) =293a(10) =349a(11) =421a(12) =489a(13) =577a(14) =665a(15) =749a(16) =861a(17) =973a(18) =1085a(19) =1201a(20) =1313a(21) =1457a(22) =1597a(23) =1741a(24) =1885a(25) =2053a(26) =2217a(27) =2377a(28) =2561a(29) =2733