Earliest sequence with a(a(n)) = 2n.

Terms

a(0) =0a(1) =3a(2) =6a(3) =2a(4) =12a(5) =7a(6) =4a(7) =10a(8) =24a(9) =11a(10) =14a(11) =18a(12) =8a(13) =15a(14) =20a(15) =26a(16) =48a(17) =19a(18) =22a(19) =34a(20) =28a(21) =23a(22) =36a(23) =42a(24) =16a(25) =27a(26) =30a(27) =50a(28) =40a(29) =31