Numbers n such that n-1 is in the 3x+1 (Colatz) trajectory of n.
2, 3, 5, 6, 9, 11, 14, 17, 18, 39, 41, 47, 54, 57, 59, 62, 71, 81, 89, 107, 108, 161, 252, 284, 378, 639, 651, 959, 977, 1368, 1439, 1823, 2159, 2430, 2735, 3239, 4103, 4617, 4859, 6155, 7289, 9233
1
No other terms less than 10^9.
T. D. Noe, Plot of 42 terms
(Mma) Collatz[n_] := NestWhileList[If[EvenQ[#], #/2, 3 # + 1] &, n, # > 1 &]; Select[Range[10000], MemberQ[Collatz[#], # - 1] &]
Cf. A070993, A276200, S000963, S000965.
nonn,more
T. D. Noe, Jan 27 2017