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 A119134 #11 Mar 31 2023 21:40:51
%S A119134 1,15,55,171,1711,5151,11175,551775,1777555,17555775,51151555,
%T A119134 115117551,1115171151,151577151715,171115717515,551171777775,
%U A119134 1175175575155,1577775175717755,5511171777517575,7571117555155155,517577151571517115,757171775575551115
%N A119134 Triangular numbers composed of digits {1,5,7}.
%H A119134 Giovanni Resta, <a href="http://www.numbersaplenty.com/tr/tr157.html">Tridigital Triangular Numbers</a>.
%F A119134 a(n) = A000217(A119135(n)). - _Tyler Busby_, Mar 31 2023
%Y A119134 Cf. A000217, A119135. See A119033 for a table of cross-references.
%K A119134 nonn,base
%O A119134 1,2
%A A119134 _Giovanni Resta_, May 10 2006
%E A119134 a(21)-a(22) from _Tyler Busby_, Mar 23 2023