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 A198590 #9 Sep 25 2023 12:39:52
%S A198590 9,19,37,77,81,149,163,309,321,325,331,597,625,643,653,663,713,715,
%T A198590 1237,1251,1285,1301,1313,1325,1339,1425,1427,1431,1433,2389,2501,
%U A198590 2573,2577,2613,2625,2627,2653,2679,2695,2851,2853,2861,2865,2867,4949,5005,5141
%N A198590 Odd numbers producing 7 odd numbers in the Collatz iteration.
%H A198590 Vincenzo Librandi, <a href="/A198590/b198590.txt">Table of n, a(n) for n = 1..1046</a>
%t A198590 Collatz[n_] := NestWhileList[If[EvenQ[#], #/2, 3 # + 1] &, n, # > 1 &]; t = {}; Do[If[Length[Select[Collatz[n], OddQ]] == 7, AppendTo[t, n]], {n, 1, 10000, 2}]; t
%t A198590 col7Q[n_]:=Length[Select[NestWhileList[If[EvenQ[#],#/2,3#+1]&,n,#>1&],OddQ]]==7; Select[ Range[1,5201,2],col7Q] (* _Harvey P. Dale_, Sep 25 2023 *)
%Y A198590 Cf. A198584.
%K A198590 nonn
%O A198590 1,1
%A A198590 _T. D. Noe_, Oct 31 2011