Earliest sequence with a(a(n))=3n.

Terms

a(0) =0a(1) =2a(2) =3a(3) =6a(4) =5a(5) =12a(6) =9a(7) =8a(8) =21a(9) =18a(10) =11a(11) =30a(12) =15a(13) =14a(14) =39a(15) =36a(16) =17a(17) =48a(18) =27a(19) =20a(20) =57a(21) =24a(22) =23a(23) =66a(24) =63a(25) =26a(26) =75a(27) =54a(28) =29a(29) =84