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 A218390 #13 Mar 27 2023 03:36:06
%S A218390 40002040,2442040424202042204020040,24202244424402424242222220,
%T A218390 200020020000400442044244444400204400200
%N A218390 Triangular numbers composed of digits {0,2,4}.
%C A218390 a(5) > 10^40.
%H A218390 Giovanni Resta, <a href="http://www.numbersaplenty.com/tr/tr024.html">Tridigital Triangular Numbers</a>.
%F A218390 a(n) = A000217(A218389(n)).
%Y A218390 Cf. A000217, A218389. See A119033 for a table of cross-references.
%K A218390 base,nonn,more
%O A218390 1,1
%A A218390 _Max Alekseyev_, Oct 27 2012