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 A119131 #12 Jul 13 2021 07:28:22
%S A119131 1,13,313,538,13741,43386,48886,991886,141415133,2880781113,
%T A119131 4721227413,19567549258,538423521386,137473211871979931561
%N A119131 Numbers k such that the k-th triangular number contains only digits {1,4,9}.
%H A119131 G. Resta, <a href="http://www.numbersaplenty.com/tr/tr149.html">Tridigital Triangular Numbers</a>
%Y A119131 Cf. A000217, A119130. See A119034 for a table of cross-references.
%K A119131 nonn,base,more
%O A119131 1,2
%A A119131 _Giovanni Resta_, May 10 2006
%E A119131 a(14) from _Max Alekseyev_, Aug 15 2013