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 A119109 #11 Apr 01 2023 09:11:28
%S A119109 1,21,91,2211,222111,22221111,229119121,291212911,2222211111,
%T A119109 222222111111,299992122991,1122921211191,22222221111111,
%U A119109 2222222211111111,21212929921912191,222222222111111111,22222222221111111111,2222222222211111111111,191929912211129991991191
%N A119109 Triangular numbers composed of digits {1,2,9}.
%H A119109 Giovanni Resta, <a href="http://www.numbersaplenty.com/tr/tr129.html">Tridigital Triangular Numbers</a>.
%F A119109 a(n) = A000217(A119110(n)). - _Tyler Busby_, Mar 31 2023
%Y A119109 Cf. A000217, A053889, A119110. See A119033 for a table of cross-references.
%K A119109 nonn,base
%O A119109 1,2
%A A119109 _Giovanni Resta_, May 10 2006
%E A119109 a(17)-a(19) from _Tyler Busby_, Mar 22 2023