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 A218397 #14 Mar 27 2023 07:28:09
%S A218397 70007020020,27272070272700,20720272227227700077007700020,
%T A218397 22200020770272007007202277772702722200
%N A218397 Triangular numbers composed of digits {0,2,7}.
%C A218397 a(5) > 10^40.
%H A218397 Giovanni Resta, <a href="http://www.numbersaplenty.com/tr/tr027.html">Tridigital Triangular Numbers</a>.
%F A218397 a(n) = A000217(A218398(n)).
%Y A218397 Cf. A000217, A218398. See A119033 for a table of cross-references.
%K A218397 base,nonn,more
%O A218397 1,1
%A A218397 _Max Alekseyev_, Oct 28 2012