Unique monotonic sequence of nonnegative integers satisfying a(a(n)) = 3n.

Terms

a(0) =0a(1) =2a(2) =3a(3) =6a(4) =7a(5) =8a(6) =9a(7) =12a(8) =15a(9) =18a(10) =19a(11) =20a(12) =21a(13) =22a(14) =23a(15) =24a(16) =25a(17) =26a(18) =27a(19) =30a(20) =33a(21) =36a(22) =39a(23) =42a(24) =45a(25) =48a(26) =51a(27) =54a(28) =55a(29) =56