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A219696 Numbers n such that the trajectory of 3n + 1 under the '3x + 1' map reaches n. 2
1, 2, 4, 8, 10, 14, 16, 20, 22, 26, 40, 44, 52, 106, 184, 206, 244, 274, 322, 526, 650, 668, 790, 866, 976, 1154, 1300, 1438, 1732, 1780, 1822, 2308, 2734, 3238, 7288 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

This sequence seems complete; there are no other terms <= 10^9. - T. D. Noe, Dec 03 2012

If on the other hand, the 3x+1 step is replaced with (3x+1)/2, the sequence is: 1, 2, 4, 8, 10, 14, 20, 22, 26, 40, 44, 206, 244, 650, 668, 866, 1154, 1822, 2308. - Robert G. Wilson v, Jan 13 2015

LINKS

Table of n, a(n) for n=1..35.

Eric Weisstein's World of Mathematics, Collatz Problem

Wikipedia, Collatz conjecture

Index entries for sequences related to 3x+1 (or Collatz) problem

EXAMPLE

For n = 4, the Collatz trajectory of 3n + 1 is (13, 40, 20, 10, 5, 16, 8, 4, 2, 1), which includes 4; thus, 4 is in the sequence.

For n = 5, the Collatz trajectory of 3n + 1 is (16, 8, 4, 2, 1), which does not include 5; thus, 5 is not in the sequence.

MATHEMATICA

Collatz[n_] := NestWhileList[If[EvenQ[#], #/2, 3 # + 1] &, n, # > 1 &]; Select[Range[10000], MemberQ[Collatz[3 # + 1], #] &] (* T. D. Noe, Dec 03 2012 *)

PROG

(Haskell)

a219696 n = a219696_list !! (n-1)

a219696_list = filter (\x -> collatz'' x == x) [1..] where

   collatz'' x = until (`elem` [1, x]) a006370 (3 * x + 1)

-- Reinhard Zumkeller, Aug 11 2014

(Python)

def ok(n):

    if n==1: return [1]

    N=3*n + 1

    l=[N, ]

    while True:

        if N%2==1: N = 3*N + 1

        else: N/=2

        l+=[N, ]

        if N<2: break

    if n in l: return 1

    return 0 # Indranil Ghosh, Apr 22 2017

CROSSREFS

Cf. A014682, A070991, A006370, A070165.

Sequence in context: A189782 A047235 A287844 * A087505 A086801 A154115

Adjacent sequences:  A219693 A219694 A219695 * A219697 A219698 A219699

KEYWORD

nonn,nice

AUTHOR

Robert C. Lyons, Nov 25 2012

EXTENSIONS

Initial 1 from Clark R. Lyons, Dec 02 2012

STATUS

approved

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Last modified October 23 05:56 EDT 2019. Contains 328335 sequences. (Running on oeis4.)