This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A221475 #13 Feb 25 2013 11:29:00
%S A221475 1,21,85,341,453,909,1365,1813,5461,7281,9669,21845,29013,29125,38835,
%T A221475 45839,54327,58197,58253,68759,77667,81491,87381,103139,116053,116501,
%U A221475 122237,154709,154737,155341,183357,232789,233013,257551,275037,310385,310669,325965
%N A221475 Odd numbers having no odd primes in their Collatz (3x+1) trajectory.
%C A221475 Sequence A196871 contains these terms and even numbers that are 2^k times terms in this sequence.
%H A221475 T. D. Noe, <a href="/A221475/b221475.txt">Table of n, a(n) for n = 1..500</a>
%H A221475 <a href="/index/3#3x1">Index entries for sequences related to 3x+1 (or Collatz) problem</a>
%t A221475 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
%Y A221475 Cf. A196871.
%K A221475 nonn
%O A221475 1,2
%A A221475 _T. D. Noe_, Feb 14 2013