a(n) = (1/3)*(s(n) + 2), where s = A026226.

Terms

a(0) =1a(1) =3a(2) =6a(3) =7a(4) =9a(5) =12a(6) =15a(7) =16a(8) =18a(9) =19a(10) =21a(11) =24a(12) =25a(13) =27a(14) =30a(15) =33a(16) =34a(17) =36a(18) =39a(19) =42a(20) =43a(21) =45a(22) =46a(23) =48a(24) =51a(25) =52a(26) =54a(27) =55a(28) =57a(29) =60