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 A087254 #9 Oct 15 2013 22:32:16
%S A087254 4,8,20,24,32,48,56,68,72,80,84,96,104,116,128,132,144,152,168,176,
%T A087254 180,192,200,212,224,228,240,260,264,272,276,288,296,308,312,320,324,
%U A087254 336,344,356,360,368,372,384,392,404,408,416,452,456,464,468,480,488,512
%N A087254 If we start the Collatz-iteration at these values, each divisible by 4, all subsequent terms in trajectory are smaller than the initial value.
%C A087254 Numbers that are not highest in any Collatz trajectory other than n. - _Jayanta Basu_, May 27 2013
%e A087254 n=104: iteration list = {104,52,26,13,40,20,10,5,16,8,4,2,1}, where initial-value = largest-term.
%t A087254 mcoll[n_]:=Max@@NestWhileList[If[EvenQ[#],#/2,3#+1] &,n,#>1 &]; t={}; Do[c=i=0; While[c!=1 && ++i<n,If[mcoll[i]==n,c=1]]; If[c==0 && mcoll[n]==n,AppendTo[t,n]],{n,4,512,4}]; t (* _Jayanta Basu_, May 27 2013 *)
%Y A087254 Cf. A025586, A087251-A087253, A222562 (with 1 and 2 prepended).
%K A087254 nonn
%O A087254 1,1
%A A087254 _Labos Elemer_, Sep 08 2003