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 A119178 #10 Mar 24 2023 03:02:53
%S A119178 28,78,2278,8778,222778,828828,887778,8272278,22227778,88278828,
%T A119178 88877778,2222277778,2882872278,8882778828,8888777778,222222777778,
%U A119178 888827778828,888887777778,22222227777778,88888277778828,88888877777778,228887828777278,272277272227278
%N A119178 Triangular numbers composed of digits {2,7,8}.
%H A119178 Giovanni Resta, <a href="http://www.numbersaplenty.com/tr/tr278.html">Tridigital Triangular Numbers</a>
%F A119178 a(n) = A000217(A119179(n)). - _Michel Marcus_, Mar 22 2023
%Y A119178 Cf. A000217, A119179. See A119033 for a table of cross-references.
%K A119178 nonn,base
%O A119178 1,1
%A A119178 _Giovanni Resta_, May 10 2006
%E A119178 a(21)-a(23) from _Tyler Busby_, Mar 22 2023