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 A119164 #13 Mar 24 2023 08:44:41
%S A119164 6,55,66,666,2556,5565,6555,26565,255255,2265256,5656566,55656525,
%T A119164 525252666,2665266555,2666226566655,6655522556665,56625555222225,
%U A119164 265556626652565,555665666566566,2626662552255225,6225222656552266,26256252552656556,652265255255252222566
%N A119164 Triangular numbers composed of digits {2,5,6}.
%H A119164 Giovanni Resta, <a href="http://www.numbersaplenty.com/tr/tr256.html">Tridigital Triangular Numbers</a>
%F A119164 a(n) = A000217(A119165(n)).
%t A119164 Flatten[Table[Select[FromDigits/@Tuples[{2,5,6},n],OddQ[Sqrt[8#+1]]&],{n,16}]] (* _Harvey P. Dale_, Nov 03 2015 *)
%Y A119164 Cf. A000217, A030484, A119165. See A119033 for a table of cross-references.
%K A119164 nonn,base
%O A119164 1,1
%A A119164 _Giovanni Resta_, May 10 2006
%E A119164 a(22)-a(23) from _Tyler Busby_, Mar 24 2023