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

Terms

a(0) =1a(1) =6a(2) =13a(3) =22a(4) =39a(5) =54a(6) =75a(7) =96a(8) =121a(9) =156a(10) =185a(11) =222a(12) =257a(13) =302a(14) =347a(15) =390a(16) =447a(17) =504a(18) =561a(19) =620a(20) =677a(21) =750a(22) =821a(23) =894a(24) =967a(25) =1052a(26) =1135a(27) =1216a(28) =1309a(29) =1396