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 A034050 #31 Feb 16 2025 08:32:36
%S A034050 3,13,31,113,131,311,1113,1131,1311,3111,11113,11131,11311,13111,
%T A034050 31111,111113,111131,111311,113111,131111,311111,1111113,1111131,
%U A034050 1111311,1113111,1131111,1311111,3111111,11111113,11111131,11111311,11113111
%N A034050 Numbers with multiplicative digital root value 3.
%C A034050 Numbers with one 3, and zero or more 1s. - _Daniel Forgues_, Oct 09 2011
%H A034050 Robert Israel, <a href="/A034050/b034050.txt">Table of n, a(n) for n = 1..10000</a>
%H A034050 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/MultiplicativeDigitalRoot.html">Multiplicative Digital Root</a>
%H A034050 <a href="/index/Ar#10-automatic">Index entries for 10-automatic sequences</a>.
%F A034050 There are n(n+1)/2 members up to 10^n, so a(n) is about 10^sqrt(2n).
%F A034050 a(m*(m-1)/2+j+1) = (10^m-1)/9 + 2*10^j for 0 <= j < m. - _Robert Israel_, Sep 27 2016
%p A034050 seq(seq((10^m-1)/9 + 2*10^j,j=0..m-1),m=1..10); # _Robert Israel_, Sep 27 2016
%t A034050 Sort[Flatten[Table[FromDigits/@Permutations[Join[{3},PadRight[{},n,1]]],{n,0,8}]]] (* _Harvey P. Dale_, Jul 16 2012 *)
%o A034050 (Python) # through 8-digit terms
%o A034050 print([int("1"*(d-i)+"3"+"1"*i) for d in range(8) for i in range(d+1)]) # _Michael S. Branicky_, Mar 13 2021
%Y A034050 Cf. A031347.
%Y A034050 Cf. A034048, A002275, A034049, A034050, A034051, A034052, A034053, A034054, A034055, A034056 (numbers having multiplicative digital roots 0-9).
%K A034050 nonn,base,easy
%O A034050 1,1
%A A034050 _Patrick De Geest_, Sep 15 1998