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 A222755 #9 Mar 05 2013 14:57:34
%S A222755 1,0,0,5,0,21,17,85,113,341,453,1365,1813,5461,7281,21845,29125,87381,
%T A222755 116501,349525,466033,1398101,1864133,5592405,7456533,22369621,
%U A222755 29826161,89478485,119304645,357913941,477218581,1431655765
%N A222755 Greatest odd number k such that difference between halving and tripling steps in Collatz (3x+1) trajectory of k is n, or 0 if there is no such k.
%C A222755 Note that a(n) <= 2^n, with equality only for n = 0.
%t A222755 Collatz[n_] := NestWhileList[If[EvenQ[#], #/2, 3 # + 1] &, n, # > 1 &]; nn = 15; t = Table[0, {nn}]; Do[c = Collatz[n]; e = Select[c, EvenQ]; diff = 2*Length[e] - Length[c]; If[diff < nn - 1, t[[diff + 2]] = n], {n, 1, 2^(nn - 1), 2}]; t
%Y A222755 Cf. A222752, A222753, A222754.
%K A222755 nonn
%O A222755 0,4
%A A222755 _T. D. Noe_, Mar 04 2013
%E A222755 a(31) added - _T. D. Noe_, Mar 05 2013