Earliest sequence with a(a(n)) = 4n.

Terms

a(0) =0a(1) =2a(2) =4a(3) =5a(4) =8a(5) =12a(6) =7a(7) =24a(8) =16a(9) =10a(10) =36a(11) =13a(12) =20a(13) =44a(14) =15a(15) =56a(16) =32a(17) =18a(18) =68a(19) =21a(20) =48a(21) =76a(22) =23a(23) =88a(24) =28a(25) =26a(26) =100a(27) =29a(28) =96a(29) =108