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 A119079 #11 Mar 22 2023 18:27:15
%S A119079 6,55,66,666,5050,5565,6555,66066,500500,600060,605550,5656566,
%T A119079 50005000,60505500,5000050000,6050055000,500000500000,605000550000,
%U A119079 50000005000000,60500005500000,66605656050060,550006056055665,555665666566566,5000000050000000
%N A119079 Triangular numbers composed of digits {0,5,6}.
%H A119079 Giovanni Resta, <a href="http://www.numbersaplenty.com/tr/tr056.html">Tridigital Triangular Numbers</a>
%F A119079 a(n) = A000217(A119080(n)). - _Michel Marcus_, Mar 22 2023
%t A119079 Rest[Select[FromDigits/@Tuples[{0,5,6},15],IntegerQ[(Sqrt[8#+1]-1)/2]&]] (* _Harvey P. Dale_, Nov 19 2013 *)
%Y A119079 Cf. A000217, A058446, A119080. See A119033 for a table of cross-references.
%K A119079 nonn,base
%O A119079 1,1
%A A119079 _Giovanni Resta_, May 10 2006
%E A119079 a(23)-a(24) from _Tyler Busby_, Mar 22 2023