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 A119122 #10 Jul 13 2021 07:28:05
%S A119122 1,2,13,621,4458,489884458,632318261,893464178,16684802253,
%T A119122 1621069592578,2502478528946,250255842662341,1543458022195053,
%U A119122 527851115257700546
%N A119122 Numbers k such that the k-th triangular number contains only digits {1,3,9}.
%H A119122 G. Resta, <a href="http://www.numbersaplenty.com/tr/tr139.html">Tridigital Triangular Numbers</a>
%Y A119122 Cf. A000217, A119121. See A119034 for a table of cross-references.
%K A119122 nonn,base,more
%O A119122 1,2
%A A119122 _Giovanni Resta_, May 10 2006
%E A119122 a(14) from _Max Alekseyev_, Oct 14 2012