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 A238193 #9 Mar 31 2014 03:54:14
%S A238193 3,21,75,151,1365,5461,14563,87381,184111,932067
%N A238193 Position of first occurrence of (4^n - 1)/3 in A238192.
%C A238193 Another version of A237661.
%C A238193 Is this (apart from the missing 1) the same as A054646? - _R. J. Mathar_, Mar 30 2014
%t A238193 Collatz[n_] := NestWhileList[If[EvenQ[#], #/2, 3 # + 1] &, n, # > 1 &]; t = Table[c = Collatz[n]; co = Select[c, OddQ]; If[Length[co] == 1, 0, co[[-2]]], {n, 10000}]; Flatten[Table[Position[t, (4^n - 1)/3, 1, 1], {n, 2, 7}]]
%Y A238193 Cf. A002450 ((4^n-1)/3), A070165 (Collatz trajectories), A238192.
%K A238193 nonn,more
%O A238193 2,1
%A A238193 _T. D. Noe_, Feb 21 2014