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 A119219 #7 Jul 13 2021 21:31:07
%S A119219 3,10,11,36,105,114,3363,3369,3648,10636,1073010,1164274,33336636,
%T A119219 33336696,107290789,1063736505,3887841963,3333333366663363
%N A119219 Numbers k such that the k-th triangular number contains only digits {5,6,7}.
%C A119219 Next term exceeds 1.4*10^16.
%H A119219 G. Resta, <a href="http://www.numbersaplenty.com/tr/tr567.html">Tridigital Triangular Numbers</a>
%Y A119219 Cf. A000217, A119218. See A119034 for a table of cross-references.
%K A119219 nonn,base,more
%O A119219 1,1
%A A119219 _Giovanni Resta_, May 10 2006