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 A176868 #8 Feb 14 2013 01:52:24
%S A176868 1,0,0,0,0,5,0,21,0,85,0,341,113,1365,453,5461,1813,21845,7281,87381,
%T A176868 29125,349525,116501,1398101,466033,5592405,1864133,22369621,7456533,
%U A176868 89478485,29826161,357913941,119304645,1431655765,477218581,5726623061,1908874353
%N A176868 Greatest odd number that requires n Collatz (3x+1) iterations to reach 1, or zero if there is no such number.
%C A176868 Both the 3x+1 steps and the halving steps are counted. a(n)=0 only for n = 1, 2, 3, 4, 6, 8, and 10.
%H A176868 T. D. Noe, <a href="/A176868/b176868.txt">Table of n, a(n) for n = 0..400</a>
%t A176868 nn = 10; t1 = {0, 0, 5, 21, 85, 341, 1365}; Do[AppendTo[t1, 4 t1[[-1]] + 1], {3*nn}]; t2 = {1, 0, 0, 0, 0, 0, 113}; Do[AppendTo[t2, 4 t2[[-1]] + 1]; AppendTo[t2, 4 t2[[-1]] + 1]; AppendTo[t2, 4 t2[[-1]] + 29], {nn}]; t = Riffle[t2, t1] (* _T. D. Noe_, Feb 14 2013 *)
%Y A176868 Cf. A176866, A176867.
%K A176868 nonn
%O A176868 0,6
%A A176868 _T. D. Noe_, Apr 27 2010