a(n) = ceiling(n^2/2).

Terms

a(0) =0a(1) =1a(2) =2a(3) =5a(4) =8a(5) =13a(6) =18a(7) =25a(8) =32a(9) =41a(10) =50a(11) =61a(12) =72a(13) =85a(14) =98a(15) =113a(16) =128a(17) =145a(18) =162a(19) =181a(20) =200a(21) =221a(22) =242a(23) =265a(24) =288a(25) =313a(26) =338a(27) =365a(28) =392a(29) =421