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 A119099 #10 Mar 19 2023 02:35:54
%S A119099 1,21,2211,222111,1422141,4244241,22221111,44241121,124212441,
%T A119099 2222211111,222222111111,22222221111111,1111114242224421,
%U A119099 2222222211111111,21414424114444221,222222222111111111,1144211112222224221,22222222221111111111,44241214122411121441
%N A119099 Triangular numbers composed of digits {1,2,4}.
%H A119099 Giovanni Resta, <a href="http://www.numbersaplenty.com/tr/tr124.html">Tridigital Triangular Numbers</a>
%F A119099 a(n) = A000217(A119100(n)). - _Michel Marcus_, Mar 19 2023
%Y A119099 Cf. A000217, A053881, A119100. See A119033 for a table of cross-references.
%K A119099 nonn,base
%O A119099 1,2
%A A119099 _Giovanni Resta_, May 10 2006
%E A119099 a(17)-a(19) from _Tyler Busby_, Mar 18 2023