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 A119186 #11 Mar 24 2023 17:14:44
%S A119186 3,6,36,55,66,666,3655,5356,5565,6555,333336,3635556,5656566,35536665,
%T A119186 56365653,56663335,63365653,655636366,53533355655,55663663653,
%U A119186 533533653555,555533363556,6653355565366,555665666566566,35356355566555653,653366333366663536656
%N A119186 Triangular numbers composed of digits {3,5,6}.
%H A119186 Giovanni Resta, <a href="http://www.numbersaplenty.com/tr/tr356.html">Tridigital Triangular Numbers</a>
%F A119186 a(n) = A000217(A119187(n)). - _Tyler Busby_, Mar 24 2023
%t A119186 Table[Select[FromDigits/@Tuples[{3,5,6},n],OddQ[Sqrt[8#+1]]&],{n,15}]//Flatten (* _Harvey P. Dale_, Aug 03 2021 *)
%Y A119186 Cf. A000217, A053945, A119187. See A119033 for a table of cross-references.
%K A119186 nonn,base
%O A119186 1,1
%A A119186 _Giovanni Resta_, May 10 2006
%E A119186 a(25)-a(26) from _Tyler Busby_, Mar 24 2023