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 A087253 #4 Oct 15 2013 22:32:16
%S A087253 1,1,0,6,1,1,0,1,0,3,0,1,12,1,0,3,1,1,0,1,1,8,0,1,3,1,0,3,1,0,0,1,1,3,
%T A087253 0,1,3,1,0,13,0,1,0,1,1,3,0,1,8,1,0,3,1,0,0,1,1,6,0,1,3,0,0,3,1,1,0,1,
%U A087253 1,3,0,1,0,1,0,14,1,1,0,1,1,0,0,1,6,1,0,3,1,1,0,1,1,0,0,1,0,1,0,3,1,1,0,1,0
%N A087253 Number of distinct initial values of various 3x+1 trajectories of which the peak-value equals 4n.
%e A087253 n=13: a(13)=12 because peak=4.13=52
%e A087253 occurs at the following 12 initial values:
%e A087253 {7,9,11,14,17,18,22,28,34,36,44,52};
%e A087253 this list differs from trajectory started at iv=52.
%e A087253 Impossible peaks of 4k form are listed in A087252
%e A087253 (corresponding to a[x]=0).
%Y A087253 Cf. A025586.
%K A087253 nonn
%O A087253 1,4
%A A087253 _Labos Elemer_, Sep 08 2003