A238192 In the Collatz (3x+1) iteration of n, the last odd number before 1, or 0 if there is no such number.

0, 0, 5, 0, 5, 5, 5, 0, 5, 5, 5, 5, 5, 5, 5, 0, 5, 5, 5, 5, 21, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 0, 5, 5, 5, 5, 5, 5, 5, 5, 5, 21, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 0, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 85, 5, 5, 5, 5, 5, 5, 5, 5, 21, 85, 5

Offset: 1

Author

T. D. Noe, Feb 21 2014

Keywords

• nonn

Comments

Another version of A237660. The only terms appearing here are 0, 5, 21, 85, ..., which is A002450 without 1.

Links

• T. D. Noe, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A002450 ((4^n-1)/3), A070165 (Collatz trajectories).

Programs

Mathematica

Collatz[n_] := NestWhileList[If[EvenQ[#], #/2, 3 # + 1] &, n, # > 1 &]; Table[c = Collatz[n]; co = Select[c, OddQ]; If[Length[co] == 1, 0, co[[-2]]], {n, 100}]