a(n) = ceiling(n^2/9).

Terms

a(0) =0a(1) =1a(2) =1a(3) =1a(4) =2a(5) =3a(6) =4a(7) =6a(8) =8a(9) =9a(10) =12a(11) =14a(12) =16a(13) =19a(14) =22a(15) =25a(16) =29a(17) =33a(18) =36a(19) =41a(20) =45a(21) =49a(22) =54a(23) =59a(24) =64a(25) =70a(26) =76a(27) =81a(28) =88a(29) =94