a(n) = (1/3)*s(n+1), where s = A026229.

Terms

a(0) =2a(1) =4a(2) =6a(3) =7a(4) =10a(5) =12a(6) =13a(7) =16a(8) =18a(9) =19a(10) =21a(11) =22a(12) =25a(13) =28a(14) =30a(15) =31a(16) =34a(17) =36a(18) =37a(19) =39a(20) =40a(21) =43a(22) =46a(23) =48a(24) =49a(25) =52a(26) =54a(27) =55a(28) =57a(29) =58