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 A228872 #5 Sep 23 2019 16:16:21
%S A228872 13,53,113,213,453,853,909,1813,3413,3637,7253,7281,13653,14549,29013,
%T A228872 29125,54613,58197,58253,116053,116501,218453,232789,233013,464213,
%U A228872 466005,466033,873813,931157,932053,1856853,1864021,1864133,3495253,3724629,3728213
%N A228872 Odd numbers producing 3 decreasing odd numbers in the Collatz (3x+1) iteration.
%C A228872 Sequence A198584 gives the first term of the Collatz sequence having exactly 3 odd numbers. This sequence is the subset of A198584 for which the second odd number is smaller than the first.
%e A228872 The number 13 has the Collatz iteration {13, 40, 20, 10, 5, 16, 8, 4, 2, 1}, which has three odd numbers in decreasing order {13, 5, 1}.
%t A228872 donQ[n_]:=Module[{od=Differences[Select[NestWhileList[If[EvenQ[#],#/2,3#+1]&,n,#>1&], OddQ]]}, Length[ od] ==2&&Max[od]<0]; Select[Range[1,373*10^4,2],donQ] (* _Harvey P. Dale_, Sep 23 2019 *)
%Y A228872 Cf. A198584 (Collatz iterations having 3 odd numbers).
%Y A228872 Cf. A228871 (Collatz iterations producing 3 out-of-order odd numbers).
%K A228872 nonn
%O A228872 1,1
%A A228872 _T. D. Noe_, Sep 12 2013