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 A119115 #13 Mar 22 2023 12:02:18
%S A119115 1,3,6,36,66,136,666,13366,333336,6631661361,113113613161,
%T A119115 663363661366,16116313316131,633161111666331,633613661136316,
%U A119115 1331116613613361,33311633636366361,663661636336331361,61163111131316636331,6313111611636313666666,1616111661666616161331131
%N A119115 Triangular numbers composed of digits {1,3,6}.
%H A119115 Giovanni Resta, <a href="http://www.numbersaplenty.com/tr/tr136.html">Tridigital Triangular Numbers</a>
%F A119115 a(n) = A000217(A119116(n)). - _Michel Marcus_, Mar 22 2023
%t A119115 Table[Select[FromDigits/@Tuples[{1,3,6},n],OddQ[Sqrt[8#+1]]&],{n,18}]//Flatten (* _Harvey P. Dale_, Sep 18 2017 *)
%Y A119115 Cf. A000217, A053893, A119116. See A119033 for a table of cross-references.
%K A119115 nonn,base
%O A119115 1,2
%A A119115 _Giovanni Resta_, May 10 2006
%E A119115 a(19)-a(21) from _Tyler Busby_, Mar 22 2023