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 A050139 #17 Oct 15 2019 11:52:42
%S A050139 2,1,12,6,3,24,28,14,7,40,20,10,5,56,60,30,15,72,36,18,9,4,92,46,23,
%T A050139 11,108,54,27,13,124,62,31,136,68,34,17,8,156,78,39,19,172,86,43,21,
%U A050139 188,94,47,200,100,50,25,216,220,110,55,232,116
%N A050139 a(1)=2; for n > 1, a(n) = floor(a(n-1)/2) if this is not among 0, a(1), ..., a(n-1); otherwise a(n) = 4*n.
%C A050139 Does this sequence contain every positive integer exactly once?
%H A050139 Michael De Vlieger, <a href="/A050139/b050139.txt">Table of n, a(n) for n = 1..10000</a>
%t A050139 Delete[#, 3] &@ Nest[Append[#1, If[FreeQ[#1, #2], #2, 4 #3]] & @@ {#, Floor[#[[-1]]/2], Length@ #} &, {2}, 59] (* _Michael De Vlieger_, Oct 06 2019 *)
%Y A050139 Cf. A050000, A050128, A050132, A050135, A050136.
%K A050139 nonn
%O A050139 1,1
%A A050139 _Clark Kimberling_