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 A218399 #18 Apr 08 2023 22:39:22
%S A218399 40470,4744740,4770740040,44747000047470770470774700707404004740
%N A218399 Triangular numbers composed of digits {0,4,7}.
%C A218399 a(5) > 10^40.
%H A218399 Giovanni Resta, <a href="http://www.numbersaplenty.com/tr/tr047.html">Tridigital Triangular Numbers</a>.
%F A218399 a(n) = A000217(A218400(n)).
%Y A218399 Cf. A000217, A218400. See A119033 for a table of cross-references.
%K A218399 base,nonn,more
%O A218399 1,1
%A A218399 _Max Alekseyev_, Oct 28 2012