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 A218400 #11 Jul 13 2021 07:24:02
%S A218400 284,3080,97680,9460126854061817544
%N A218400 Numbers k such that the k-th triangular number contains only digits {0,4,7}.
%C A218400 No other terms below 1.4*10^20.
%H A218400 G. Resta, <a href="http://www.numbersaplenty.com/tr/tr047.html">Tridigital Triangular Numbers</a>
%Y A218400 Cf. A000217, A218399. See A119034 for a table of cross-references.
%K A218400 base,nonn,more
%O A218400 1,1
%A A218400 _Max Alekseyev_, Oct 28 2012