a(n) = ceiling(n^2/3).

Terms

a(0) =0a(1) =1a(2) =2a(3) =3a(4) =6a(5) =9a(6) =12a(7) =17a(8) =22a(9) =27a(10) =34a(11) =41a(12) =48a(13) =57a(14) =66a(15) =75a(16) =86a(17) =97a(18) =108a(19) =121a(20) =134a(21) =147a(22) =162a(23) =177a(24) =192a(25) =209a(26) =226a(27) =243a(28) =262a(29) =281