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 A199818 #12 Mar 15 2023 14:02:15
%S A199818 43,131,133,397,405,433,435,441,475,1237,1251,1285,1301,1313,1325,
%T A199818 1339,1425,1427,1431,1433,3861,3925,3939,3941,3981,4017,4019,4043,
%U A199818 4277,4293,4297,4301,11605,11829,12053,12131,12133,12853,12885,12893,12913,12931,36181
%N A199818 Odd numbers producing 20 even numbers in the Collatz iteration.
%C A199818 For n <10000000, more terms: 36405, 38677, 38741, 38797, 38833, 38835, 116053, 116501, 349525.
%C A199818 See row 20 of A199636. There are A131450(20) = 52 terms. - _T. D. Noe_, Nov 18 2011
%t A199818 Collatz[n_] := NestWhileList[If[EvenQ[#], #/2, 3 # + 1] &, n, # > 1 &]; t = {}; Do[If[Length[Select[Collatz[n], EvenQ]] == 20, AppendTo[t, n]], {n, 1, 100000, 2}]; t
%Y A199818 Cf. A199636.
%K A199818 nonn
%O A199818 1,1
%A A199818 _Vincenzo Librandi_, Nov 12 2011