(A038590-1)/6.

Terms

a(0) =0a(1) =1a(2) =2a(3) =3a(4) =5a(5) =6a(6) =7a(7) =9a(8) =10a(9) =12a(10) =14a(11) =15a(12) =16a(13) =18a(14) =20a(15) =21a(16) =23a(17) =25a(18) =27a(19) =28a(20) =31a(21) =33a(22) =35a(23) =37a(24) =39a(25) =40a(26) =42a(27) =44a(28) =45a(29) =47