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 A050020 #17 Oct 05 2019 11:32:22
%S A050020 1,10,5,2,20,200,100,50,25,12,6,3,30,15,7,70,35,17,8,4,40,400,4000,
%T A050020 2000,1000,500,250,125,62,31,310,155,77,38,19,9,90,45,22,11,110,55,27,
%U A050020 13,130,65,32,16,160,80,800,8000,80000,40000,20000
%N A050020 a(n) = floor(a(n-1)/2) if this is positive and not yet in the sequence, otherwise a(n) = 10*a(n-1).
%H A050020 Ivan Neretin, <a href="/A050020/b050020.txt">Table of n, a(n) for n = 1..10000</a>
%t A050020 Rest@Nest[Append[#, If[MemberQ[#, c = Quotient[#[[-1]], 2]], 10*#[[-1]], c]] &, {0, 1}, 54] (* _Ivan Neretin_, Sep 06 2015 *)
%o A050020 (PARI) first(n)=my(v=vector(n), t); v[1]=1; for(i=2, n, t=v[i-1]\2; if(t<2, v[i]=10*v[i-1]; next); for(j=1, i-1, if(v[j]==t, v[i]=10*v[i-1]; next(2))); v[i]=t); v \\ _Charles R Greathouse IV_, Jul 31 2016
%Y A050020 Cf. A050000 and references therein.
%K A050020 nonn,easy
%O A050020 1,2
%A A050020 _Clark Kimberling_