Numbers of the form 3^i * (3k+1).

Terms

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