a(n) = ceiling(n^2/7).

Terms

a(0) =0a(1) =1a(2) =1a(3) =2a(4) =3a(5) =4a(6) =6a(7) =7a(8) =10a(9) =12a(10) =15a(11) =18a(12) =21a(13) =25a(14) =28a(15) =33a(16) =37a(17) =42a(18) =47a(19) =52a(20) =58a(21) =63a(22) =70a(23) =76a(24) =83a(25) =90a(26) =97a(27) =105a(28) =112a(29) =121