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 A218401 #18 Mar 27 2023 03:36:32
%S A218401 990,90779090007770790,79909779970709079970099779979097777700
%N A218401 Triangular numbers composed of digits {0,7,9}.
%C A218401 a(4) > 10^40.
%H A218401 Giovanni Resta, <a href="http://www.numbersaplenty.com/tr/tr079.html">Tridigital Triangular Numbers</a>.
%F A218401 a(n) = A000217(A218402(n)).
%Y A218401 Cf. A000217, A218402. See A119033 for a table of cross-references.
%K A218401 base,nonn,more,hard,bref
%O A218401 1,1
%A A218401 _Max Alekseyev_, Oct 28 2012