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 A308149 #19 Aug 11 2019 00:03:24 %S A308149 1,2,4,8,16,21,32,42,64,75,84,85,113,128,150,151,168,170,201,226,227, %T A308149 256,267,300,301,302,336,340,341,401,402,403,423,452,453,454,475,512, %U A308149 534,535,537,600,602,604,605,633,635,672,680,682,713,715,802,803,804,805,806,846,847,891,904,906,908,909,950,951,953,955 %N A308149 Positive integers with Collatz trajectories that do not include the number 5. %C A308149 A consequence of the Collatz conjecture is that, after a(5), this would be equivalent to the list of positive integers with Collatz trajectories that include the number 32. %H A308149 Anonymous, <a href="http://archive.fo/IyVtW">Archive of /sci/ - Conjecture</a>. %H A308149 <a href="/index/3#3x1">Index entries for sequences related to 3x+1 (or Collatz) problem</a> %e A308149 21 is a term because its Collatz trajectory, given in A033481, does not include 5. %t A308149 collatzTrajectory[n_] := NestWhileList[If[EvenQ[#], #/2, 3 # + 1] &, n, # > 1 &]; Select[Range[1000], Not[MemberQ[collatzTrajectory[#], 5]] &] (* _Alonso del Arte_, May 25 2019 *) %Y A308149 Cf. A006370, A033481, A070165. %Y A308149 A000079 is a subsequence. %K A308149 nonn,easy %O A308149 1,2 %A A308149 _Nathan Bumber_, May 14 2019