a(n) = (1/3)*(s(n) + 1), where s = A026231.

Terms

a(0) =3a(1) =6a(2) =7a(3) =9a(4) =12a(5) =15a(6) =16a(7) =18a(8) =19a(9) =21a(10) =24a(11) =25a(12) =27a(13) =30a(14) =33a(15) =34a(16) =36a(17) =39a(18) =42a(19) =43a(20) =45a(21) =46a(22) =48a(23) =51a(24) =52a(25) =54a(26) =55a(27) =57a(28) =60a(29) =61