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 A119199 #7 Jul 13 2021 21:29:30
%S A119199 2,3,8,11,36,816,1368,1388,2816,89408,445788,11284236,26064036,
%T A119199 35598783,281768343,1414001388,8594921016,117759639662036,
%U A119199 887689581254543,894084295764816,2581990177997183,2696788235658743
%N A119199 Numbers k such that the k-th triangular number contains only digits {3,6,9}.
%H A119199 G. Resta, <a href="http://www.numbersaplenty.com/tr/tr369.html">Tridigital Triangular Numbers</a>
%Y A119199 Cf. A000217, A119198. See A119034 for a table of cross-references.
%K A119199 nonn,base
%O A119199 1,1
%A A119199 _Giovanni Resta_, May 10 2006