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

Terms

a(0) =1a(1) =4a(2) =8a(3) =13a(4) =22a(5) =30a(6) =41a(7) =52a(8) =65a(9) =83a(10) =98a(11) =117a(12) =135a(13) =158a(14) =181a(15) =203a(16) =232a(17) =261a(18) =290a(19) =320a(20) =349a(21) =386a(22) =422a(23) =459a(24) =496a(25) =539a(26) =581a(27) =622a(28) =669a(29) =713