A078440 Numbers n with property that n is not a power of 2 and the finite sequence n, f(n), f(f(n)), ...., 1 in the Collatz (or 3x + 1) problem contains exactly one prime. (The earliest "1" is meant.)

21, 42, 84, 85, 168, 170, 336, 340, 341, 453, 672, 680, 682, 906, 909, 1344, 1360, 1364, 1365, 1812, 1813, 1818, 2688, 2720, 2728, 2730, 3624, 3626, 3636, 5376, 5440, 5456, 5460, 5461, 7248, 7252, 7272, 7281, 9669

Offset

1,1

Comments

f(n) = n/2 if n is even, = 3n + 1 if n is odd. Powers 2^n trivially have exactly one prime in n, f(n), f(f(n)), ..., 2, 1, namely 2 and so are excluded from the sequence.

A055509(a(n)) = 0; A078350(a(n)) <= 1.

Links

Reinhard Zumkeller, Table of n, a(n) for n = 1..1000

Eric Weisstein's World of Mathematics, Collatz Problem

Wikipedia, Collatz conjecture

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

Example

n, f(n), f(f(n)), .... for n = 21 is: 21, 64, 32, 16, 8, 4, 2, 1, which has exactly one prime, that is, 2. Hence 21 belongs to the sequence.

Mathematica

f[n_] := n/2 /; Mod[n, 2] == 0 f[n_] := 3 n + 1 /; Mod[n, 2] == 1 g[n_] := Module[{i, p}, i = n; p = 0; While[i > 1, If[PrimeQ[i], p = p + 1]; i = f[i]]; p]; Select[Range[10^4], g[ # ] == 1 && ! IntegerQ[Log[2, # ]] &]
pQ[n_]:=Count[NestWhileList[If[EvenQ[#], #/2, 3#+1]&, n, #>1&], _?PrimeQ] == 1; With[ {nn=10000}, Complement[Select[Range[nn], pQ], 2^Range[Floor[ Log[ 2, nn]]]]] (* Harvey P. Dale, Oct 19 2011 *)

Prog

(Haskell)
a078440 n = a078440_list !! (n-1)
a078440_list = filter notbp a196871_list where
   notbp x = m > 0 && x > 1 || m == 0 && notbp x' where
      (x', m) = divMod x 2
-- Reinhard Zumkeller, Oct 08 2011

Crossrefs

A006370; subsequence of A196871 (with binary powers).

Sequence in context: A001682 A180963 A355406 * A175805 A039344 A043167

Adjacent sequences: A078437 A078438 A078439 * A078441 A078442 A078443

Keyword

nonn

Author

Joseph L. Pe, Dec 31 2002

Status

approved