Number of lattice points inside circle of radius n is 4(a(n)+n)-3.

Terms

a(0) =0a(1) =1a(2) =4a(3) =8a(4) =13a(5) =22a(6) =30a(7) =41a(8) =54a(9) =67a(10) =83a(11) =98a(12) =117a(13) =139a(14) =160a(15) =183a(16) =206a(17) =234a(18) =263a(19) =292a(20) =322a(21) =357a(22) =390a(23) =424a(24) =461a(25) =502a(26) =545a(27) =585a(28) =626a(29) =673