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 A224540 #14 Jun 05 2013 00:51:36
%S A224540 1,2,4,12,36,106,249,613,1732,8028,23348,69370,210807,634839,1893582,
%T A224540 5686389,17031777,51073675,153185957,459516225,1378707224,4135278456
%N A224540 Number of numbers k such that all terms of the Collatz (3x+1) iteration of k are <= 3^n.
%e A224540 For n = 3, the twelve k are 1, 2, 3, 4, 5, 6, 8, 10, 12, 16, 20, and 24.
%t A224540 Collatz[n_] := NestWhileList[If[EvenQ[#], #/2, 3 # + 1] &, n, # > 1 &]; Table[Select[Range[3^n], Max[Collatz[#]] <= 3^n &], {n, 8}]
%Y A224540 Cf. A006884, A006885, A095384, A224538.
%K A224540 nonn,more
%O A224540 0,2
%A A224540 _T. D. Noe_, Apr 24 2013
%E A224540 a(20)-a(21) from _Donovan Johnson_, Jun 05 2013