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 A039502 #9 Dec 19 2015 12:57:02
%S A039502 17,21,23,25,31,33,34,37,41,42,45,46,49,50,55,61,62,66,67,68,73,74,82,
%T A039502 83,84,90,91,92,98,99,100,109,110,111,117,122,123,124,131,132,134,136,
%U A039502 146,147,148,153,163,164,165,166,168,175,179,180,182,184,185,195,196
%N A039502 Iterations of "n->n/2 if n even, n->3n-1 if n odd" (A001281) starting at these numbers reach 17.
%t A039502 colln[n_]:= NestWhile[If[EvenQ[#], #/2, 3#-1] &, n, FreeQ[{1, 5, 17}, #] &]; Select[Range[196], colln[#] == 17 &] (* _Jayanta Basu_, Jun 06 2013 *)
%Y A039502 Cf. A037084, A039500-A039505.
%K A039502 nonn,easy
%O A039502 1,1
%A A039502 _Christian G. Bower_, Feb 15 1999