A221475 Odd numbers having no odd primes in their Collatz (3x+1) trajectory.

1, 21, 85, 341, 453, 909, 1365, 1813, 5461, 7281, 9669, 21845, 29013, 29125, 38835, 45839, 54327, 58197, 58253, 68759, 77667, 81491, 87381, 103139, 116053, 116501, 122237, 154709, 154737, 155341, 183357, 232789, 233013, 257551, 275037, 310385, 310669, 325965

Offset: 1

Author

T. D. Noe, Feb 14 2013

Keywords

• nonn

Comments

Sequence A196871 contains these terms and even numbers that are 2^k times terms in this sequence.

Links

• T. D. Noe, Table of n, a(n) for n = 1..500
• Index entries for sequences related to 3x+1 (or Collatz) problem

Crossrefs

Cf. A196871.

Programs

Mathematica

Collatz[n_] := NestWhileList[If[EvenQ[#], #/2, 3 # + 1] &, n, # > 1 &]; t = {1}; n = 1; While[Length[t] < 50, n = n + 2; If[Union[Drop[PrimeQ[Collatz[n]], -2]] == {False}, AppendTo[t, n]]]; t