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

Terms

a(0) =0a(1) =0a(2) =1a(3) =3a(4) =5a(5) =8a(6) =12a(7) =16a(8) =21a(9) =27a(10) =33a(11) =40a(12) =48a(13) =56a(14) =65a(15) =75a(16) =85a(17) =96a(18) =108a(19) =120a(20) =133a(21) =147a(22) =161a(23) =176a(24) =192a(25) =208a(26) =225a(27) =243a(28) =261a(29) =280