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 A119216 #16 Mar 27 2023 01:29:00
%S A119216 78,8778,448878,887778,7478778,88877778,8888777778,44847878778,
%T A119216 747488474778,888887777778,88888877777778,8888888777777778,
%U A119216 888888887777777778,7778777484477877878,88888888877777777778,7444484774887874784778,8888888888777777777778
%N A119216 Triangular numbers composed of digits {4,7,8}.
%H A119216 Giovanni Resta, <a href="http://www.numbersaplenty.com/tr/tr478.html">Tridigital Triangular Numbers</a>.
%F A119216 a(n) = A000217(A119217(n)). - _Tyler Busby_, Mar 26 2023
%t A119216 Table[Select[FromDigits/@Tuples[{4,7,8},n],OddQ[Sqrt[8#+1]]&],{n,20}]//Flatten (* The program will take a long time to run. *) (* _Harvey P. Dale_, Oct 15 2017 *)
%Y A119216 Cf. A000217, A053963, A119217. See A119033 for a table of cross-references.
%K A119216 nonn,base
%O A119216 1,1
%A A119216 _Giovanni Resta_, May 10 2006
%E A119216 a(16)-a(17) from _Tyler Busby_, Mar 26 2023