login
A238192
In the Collatz (3x+1) iteration of n, the last odd number before 1, or 0 if there is no such number.
3
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
1,3
COMMENTS
Another version of A237660. The only terms appearing here are 0, 5, 21, 85, ..., which is A002450 without 1.
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}]
CROSSREFS
Cf. A002450 ((4^n-1)/3), A070165 (Collatz trajectories).
Sequence in context: A261839 A091672 A144702 * A156716 A055510 A200397
KEYWORD
nonn
AUTHOR
T. D. Noe, Feb 21 2014
STATUS
approved