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 A119125 #15 Jun 20 2023 21:02:39
%S A119125 1,6,66,666,6441,41616,146611,464166,1464616,4114146,14116641,
%T A119125 664611111,1164441411,16646464416,41641661166,66114661161,
%U A119125 144161141446,161646441166,1464464661166,4464411144441,44416146616416,61116114144646,466141616644641,1116644466666141,1161666441164166
%N A119125 Triangular numbers composed of digits {1,4,6}.
%H A119125 Giovanni Resta, <a href="http://www.numbersaplenty.com/tr/tr146.html">Tridigital Triangular Numbers</a>
%F A119125 a(n) = A000217(A119126(n)). - _Michel Marcus_, Mar 22 2023
%t A119125 Table[Select[FromDigits/@Tuples[{1,4,6},n],OddQ[Sqrt[8#+1]]&],{n,16}]//Flatten (* _Harvey P. Dale_, Jun 20 2023 *)
%Y A119125 Cf. A000217, A027676, A119126. See A119033 for a table of cross-references.
%K A119125 nonn,base
%O A119125 1,2
%A A119125 _Giovanni Resta_, May 10 2006
%E A119125 a(22)-a(24) from _Tyler Busby_, Mar 22 2023
%E A119125 a(25) from _Harvey P. Dale_, Jun 20 2023