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 A119097 #13 Mar 31 2023 15:10:26
%S A119097 1,3,21,231,2211,3321,21321,32131,222111,232221,11132121,22221111,
%T A119097 22321221,2222211111,2223211221,12221332311,222222111111,222232111221,
%U A119097 1121113113121,1231322213121,22222221111111,22222321111221,2222222211111111,2222223211111221
%N A119097 Triangular numbers composed of digits {1,2,3}.
%H A119097 Giovanni Resta, <a href="http://www.numbersaplenty.com/tr/tr123.html">Tridigital Triangular Numbers</a>.
%F A119097 a(n) = A000217(A119098(n)). - _Tyler Busby_, Mar 31 2023
%t A119097 Flatten[Table[Select[FromDigits/@Tuples[{1,2,3},n],IntegerQ[(Sqrt[ 8#+1]- 1)/2]&],{n,14}]] (* _Harvey P. Dale_, May 16 2014 *)
%Y A119097 Cf. A000217, A030174, A119098. See A119033 for a table of cross-references.
%K A119097 nonn,base
%O A119097 1,2
%A A119097 _Giovanni Resta_, May 10 2006
%E A119097 a(23)-a(24) from _Tyler Busby_, Mar 23 2023