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 A224538 #16 Jun 05 2013 00:51:19
%S A224538 1,4,49,340,4235,39706,397068,3970918,39523168,395436300,3953296865
%N A224538 Number of numbers k such that all terms of the Collatz (3x+1) iteration of k are <= 10^n.
%e A224538 For n = 1, the four k are 1, 2, 4, and 8.
%t A224538 Collatz[n_] := NestWhileList[If[EvenQ[#], #/2, 3 # + 1] &, n, # > 1 &]; Table[Length[Select[Range[10^n], Max[Collatz[#]] <= 10^n &]], {n, 0, 5}]
%Y A224538 Cf. A006884, A006885, A095384, A222291, A224540.
%K A224538 nonn,more
%O A224538 0,2
%A A224538 _T. D. Noe_, Apr 24 2013
%E A224538 a(10) from _Donovan Johnson_, Jun 05 2013