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 A133195 #21 Feb 09 2022 19:40:54
%S A133195 0,3,6,9,39,69,99,399,699,999,3999,6999,9999,39999,69999,99999,399999,
%T A133195 699999,999999,3999999,6999999,9999999,39999999,69999999,99999999,
%U A133195 399999999,699999999,999999999,3999999999,6999999999,9999999999,39999999999,69999999999
%N A133195 Smallest number whose sum of digits is 3n.
%H A133195 <a href="/index/Rec">Index entries for linear recurrences with constant coefficients</a>, signature (1,0,10,-10).
%F A133195 a(n) = 1/3 * A133201(n).
%F A133195 a(n) = a(n-1)+10*a(n-3)-10*a(n-4). G.f.: 3*x*(x^2+x+1) / ((x-1)*(10*x^3-1)). [_Colin Barker_, Feb 01 2013]
%t A133195 LinearRecurrence[{1,0,10,-10},{0,3,6,9},40] (* _Harvey P. Dale_, Oct 01 2018 *)
%o A133195 (Python)
%o A133195 def a(n): q, r = divmod(3*n, 9); return int(str(r) + "9"*q)
%o A133195 print([a(n) for n in range(31)]) # _Michael S. Branicky_, Feb 07 2022
%Y A133195 Cf. A133201.
%K A133195 nonn,base,easy
%O A133195 0,2
%A A133195 _Paul Curtz_, Oct 09 2007