If k appears, 3k does not.

Terms

a(0) =1a(1) =2a(2) =4a(3) =5a(4) =7a(5) =8a(6) =9a(7) =10a(8) =11a(9) =13a(10) =14a(11) =16a(12) =17a(13) =18a(14) =19a(15) =20a(16) =22a(17) =23a(18) =25a(19) =26a(20) =28a(21) =29a(22) =31a(23) =32a(24) =34a(25) =35a(26) =36a(27) =37a(28) =38a(29) =40