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 A119110 #8 Jul 13 2021 01:24:54
%S A119110 1,6,13,66,666,6666,21406,24133,66666,666666,774586,1498613,6666666,
%T A119110 66666666,205975386,666666666,6666666666,66666666666,619564221386,
%U A119110 666666666666,6650249918806,6666666666666,7654017627258
%N A119110 Numbers k such that the k-th triangular number contains only digits {1,2,9}.
%H A119110 G. Resta, <a href="http://www.numbersaplenty.com/tr/tr129.html">Tridigital Triangular Numbers</a>
%Y A119110 Cf. A000217, A119109. See A119034 for a table of cross-references.
%K A119110 nonn,base
%O A119110 1,2
%A A119110 _Giovanni Resta_, May 10 2006