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 A119236 #15 Mar 27 2023 22:36:27
%S A119236 78,8778,887778,88877778,8888777778,777887999778,888887777778,
%T A119236 7798988788878,88888877777778,8888888777777778,888888887777777778,
%U A119236 88888888877777777778,8888888888777777777778,78879897887889899897778,888888888887777777777778,88888888888877777777777778
%N A119236 Triangular numbers composed of digits {7,8,9}.
%H A119236 Giovanni Resta, <a href="http://www.numbersaplenty.com/tr/tr789.html">Tridigital Triangular Numbers</a>.
%F A119236 a(n) = A000217(A119237(n)). - _Michel Marcus_, Mar 27 2023
%Y A119236 Cf. A000217, A058472, A119237. See A119033 for a table of cross-references.
%K A119236 nonn,base
%O A119236 1,1
%A A119236 _Giovanni Resta_, May 10 2006
%E A119236 a(15)-a(16) from _Tyler Busby_, Mar 27 2023