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 A119101 #10 Mar 22 2023 10:53:36
%S A119101 1,15,21,55,1225,2211,5151,15225,21115,22155,151525,222111,255255,
%T A119101 11551221,12552555,21115251,22221111,51151555,111221155,251115255,
%U A119101 2222211111,2552515525,111251152551,122515222515,222222111111,1125152255151,2522551111515,5255121522225
%N A119101 Triangular numbers composed of digits {1,2,5}.
%H A119101 Giovanni Resta, <a href="http://www.numbersaplenty.com/tr/tr125.html">Tridigital Triangular Numbers</a>
%F A119101 a(n) = A000217(A119102(n)). - _Michel Marcus_, Mar 22 2023
%Y A119101 Cf. A000217, A031153, A119102. See A119033 for a table of cross-references.
%K A119101 nonn,base
%O A119101 1,2
%A A119101 _Giovanni Resta_, May 10 2006
%E A119101 a(26)-a(28) from _Tyler Busby_, Mar 22 2023