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 A119202 #14 Aug 28 2023 13:11:27
%S A119202 6,45,55,66,465,666,4465,4656,5565,6555,46665,544446,4465566,5456556,
%T A119202 5656566,64564566,4565545455565,4664644554466,56466665456545,
%U A119202 555445466565465,555445666545666,555665666566566,4446456564566646,6666664664655666,46566644555566656
%N A119202 Triangular numbers composed of digits {4,5,6}.
%H A119202 Giovanni Resta, <a href="http://www.numbersaplenty.com/tr/tr456.html">Tridigital Triangular Numbers</a>.
%F A119202 a(n) = A000217(A119203(n)). - _Tyler Busby_, Mar 25 2023
%t A119202 Table[Select[FromDigits/@Tuples[{4,5,6},n],OddQ[Sqrt[8#+1]]&],{n,17}]//Flatten (* _Harvey P. Dale_, Aug 28 2023 *)
%Y A119202 Cf. A000217, A030176, A119203. See A119033 for a table of cross-references.
%K A119202 nonn,base
%O A119202 1,1
%A A119202 _Giovanni Resta_, May 10 2006
%E A119202 a(24)-a(25) from _Tyler Busby_, Mar 25 2023