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 A119208 #16 Mar 27 2023 05:46:24
%S A119208 45,55,595,5995,49455,55945,594595,5559445,555594445,9955945495,
%T A119208 55555944445,95444955595,5555559444445,9545454599955,499594959455595,
%U A119208 549999555454495,555555594444445,55555555944444445,5555555559444444445,5999995494459499455
%N A119208 Triangular numbers composed of digits {4,5,9}.
%H A119208 Giovanni Resta, <a href="http://www.numbersaplenty.com/tr/tr459.html">Tridigital Triangular Numbers</a>.
%F A119208 a(n) = A000217(A119209(n)). - _Tyler Busby_, Mar 25 2023
%t A119208 Select[Flatten[Table[FromDigits/@Tuples[{4,5,9},n],{n,17}]],OddQ[ Sqrt[ 8#+1]]&] (* The program will take a long time to run. *) (* _Harvey P. Dale_, May 23 2021 *)
%Y A119208 Cf. A000217, A053955, A119209. See A119033 for a table of cross-references.
%K A119208 nonn,base
%O A119208 1,1
%A A119208 _Giovanni Resta_, May 10 2006
%E A119208 a(19)-a(20) from _Tyler Busby_, Mar 25 2023