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 A119035 #15 Mar 31 2023 06:14:07
%S A119035 1,3,10,300,3003,10011,303031,1010331,1030330,1313010,101111310,
%T A119035 113033130,1033010331,11031100311,11033031331,31031010003,
%U A119035 111111101310,313313301003,10113301131300,3330130113300130003,1113011003000100031311,11013033300110103011310
%N A119035 Triangular numbers composed of digits {0,1,3}.
%H A119035 Giovanni Resta, <a href="http://www.numbersaplenty.com/tr/tr013.html">Tridigital Triangular Numbers</a>.
%F A119035 a(n) = A000217(A119036(n)). - _Tyler Busby_, Mar 31 2023
%t A119035 Rest[Select[FromDigits/@Tuples[{0, 1, 3}, 10], IntegerQ[(Sqrt[8 # + 1] - 1)/2] &]] (* _Vincenzo Librandi_, Dec 18 2015 *)
%o A119035 (Magma) [t: n in [1..2*10^7] | Set(Intseq(t)) subset {0,1,3} where t is n*(n+1) div 2]; // _Vincenzo Librandi_, Dec 18 2015
%Y A119035 Cf. A000217, A119036. See A119033 for a table of cross-references.
%K A119035 nonn,base
%O A119035 1,2
%A A119035 _Giovanni Resta_, May 10 2006
%E A119035 a(21)-a(22) from _Tyler Busby_, Mar 22 2023