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 A119123 #11 Mar 23 2023 19:16:45
%S A119123 1,15,45,55,5151,44551,45451,1551441,51141441,51151555,114511411,
%T A119123 4141455555,44144455545,5541155454555,54145555454451,114514415144451,
%U A119123 455441144111551,1151514555151555,5144441415145515,45441555441511411,444441144541554111,45545414514441444141
%N A119123 Triangular numbers composed of digits {1,4,5}.
%H A119123 Giovanni Resta, <a href="http://www.numbersaplenty.com/tr/tr145.html">Tridigital Triangular Numbers</a>
%F A119123 a(n) = A000217(A119124(n)). - _Michel Marcus_, Mar 23 2023
%t A119123 Select[Flatten[Table[FromDigits/@Tuples[{1,4,5},n],{n,17}]],OddQ[Sqrt[8#+1]]&] (* _Harvey P. Dale_, Feb 19 2023 *)
%Y A119123 Cf. A000217, A053897, A119124. See A119033 for a table of cross-references.
%K A119123 nonn,base
%O A119123 1,2
%A A119123 _Giovanni Resta_, May 10 2006
%E A119123 a(21)-a(22) from _Tyler Busby_, Mar 23 2023