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 A119103 #13 Mar 16 2023 04:31:52
%S A119103 1,6,21,66,666,2211,6216,212226,222111,1222266,1261666,16626261,
%T A119103 22121226,22221111,121111266,126222216,661261161,2162616261,
%U A119103 2221211226,2222211111,21166222626,66126661611,166122226621,216262612611,222212111226,222222111111,22162261612611
%N A119103 Triangular numbers composed of digits {1,2,6}.
%H A119103 Giovanni Resta, <a href="http://www.numbersaplenty.com/tr/tr126.html">Tridigital Triangular Numbers</a>
%F A119103 a(n) = A000217(A119104(n)). - _Michel Marcus_, Mar 16 2023
%t A119103 Flatten[Table[Select[FromDigits/@Tuples[{1,2,6},n],OddQ[Sqrt[8#+1]]&],{n,12}]] (* _Harvey P. Dale_, Mar 07 2015 *)
%Y A119103 Cf. A000217, A053883, A119104. See A119033 for a table of cross-references.
%K A119103 nonn,base
%O A119103 1,2
%A A119103 _Giovanni Resta_, May 10 2006
%E A119103 a(24)-a(27) from _Tyler Busby_, Mar 14 2023