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 A063013 #30 May 01 2025 13:23:58
%S A063013 0,1,20,21,300,301,320,321,4000,4001,4020,4021,4300,4301,4320,4321,
%T A063013 50000,50001,50020,50021,50300,50301,50320,50321,54000,54001,54020,
%U A063013 54021,54300,54301,54320,54321,600000,600001,600020,600021,600300,600301,600320,600321
%N A063013 Numbers where k-th digit from right is either 0 or k.
%C A063013 a(511) = 987654321 is the last term.
%H A063013 Alois P. Heinz, <a href="/A063013/b063013.txt">Table of n, a(n) for n = 0..511</a>
%F A063013 a(n) = a(n-2^floor(log_2(n))) + floor(log_2(n)+1)*10^floor(log_2(n)).
%F A063013 a(n) = Sum_{k>=0} A030308(n,k)*A053541(k+1). - _Philippe Deléham_, Oct 15 2011
%e A063013 a(11) = 4021 since 11 is written as 1011 in binary and the 1's can then be replaced by the relevant digits.
%p A063013 a:= n-> (l-> parse(cat(seq(l[-i]*(1+nops(l)-i),
%p A063013 i=1..nops(l)))))(convert(n, base, 2)):
%p A063013 seq(a(n), n=0..2^6-1); # _Alois P. Heinz_, Oct 29 2018
%t A063013 FromDigits /@ (Range[9,1,-1]*# & /@ IntegerDigits[Range[0, 511], 2, 9]) (* _Giovanni Resta_, Oct 28 2018 *)
%o A063013 (PARI) a(n) = fromdigits(Vec(Pol(binary(2*n))')); \\ _Alan Michael Gómez Calderón_, May 01 2025
%Y A063013 Cf. A063012, A053541.
%K A063013 base,nonn,fini,full
%O A063013 0,3
%A A063013 _Henry Bottomley_, Jul 04 2001