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 A253720 #19 Nov 13 2015 12:03:13
%S A253720 1,2,5,3,4,2,6,5,7,5,18,5,6,3,8,4,7,4,19,6,5,2,7,4,20,6,8,19,3,5,16,
%T A253720 18,21,7,15,4,20,5,9,8,17,18,10,8,8,5,10,18,21,6,3,7,9,3,5,19,11,8,14,
%U A253720 8,6,4,10,17,22,7
%N A253720 a(n) = length of row n in A253676 and A254068, assuming the 3x+1 (or Collatz) conjecture.
%F A253720 For n>1, k>=1, a(n) = a((8+(3*n-2)*4^k)/12).
%t A253720 (* Row lengths of A253676 and A254068: *)
%t A253720 v[n_] := IntegerExponent[n, 2]; f[x_] := (3*x + 1)/2^v[3*x + 1]; s[x_] := (3 + (3/2)^v[1 + f[x]]*(1 + f[x]))/6; A253676[n_] := NestWhileList[s[4*# - 3] &, n, # > 1 &]; Table[Length[A253676[n]], {n, 1, 66}]
%Y A253720 Cf. A253676, A254068.
%K A253720 nonn
%O A253720 1,2
%A A253720 _L. Edson Jeffery_, May 02 2015