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 A119106 #8 Jul 13 2021 01:24:45
%S A119106 1,6,18,58,66,381,493,666,1193,6666,12018,66666,156058,379941,585281,
%T A119106 666666,6666666,15603058,66666666,210319381,666666666,4933998806,
%U A119106 6666666666,66666666666,185864319933,666666666666,6666666666666
%N A119106 Numbers k such that the k-th triangular number contains only digits {1,2,7}.
%H A119106 G. Resta, <a href="http://www.numbersaplenty.com/tr/tr127.html">Tridigital Triangular Numbers</a>
%Y A119106 Cf. A000217, A119105. See A119034 for a table of cross-references.
%K A119106 nonn,base
%O A119106 1,2
%A A119106 _Giovanni Resta_, May 10 2006