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 A119069 #9 Mar 22 2023 18:27:27
%S A119069 3,300,903,990,3003,33930,39903,339900,930930,39033030,990903903,
%T A119069 3909003990,3993990000,33003300903,39999939903,90993993300,
%U A119069 3390003090090,99990099000330,90009933903093903,399939399933939330,303300903903330099003,3309000900300303999300
%N A119069 Triangular numbers composed of digits {0,3,9}.
%H A119069 Giovanni Resta, <a href="http://www.numbersaplenty.com/tr/tr039.html">Tridigital Triangular Numbers</a>
%F A119069 a(n) = A000217(A119070(n)). - _Michel Marcus_, Mar 22 2023
%Y A119069 Cf. A000217, A058434, A119070. See A119033 for a table of cross-references.
%K A119069 nonn,base
%O A119069 1,1
%A A119069 _Giovanni Resta_, May 10 2006
%E A119069 a(21)-a(22) from _Tyler Busby_, Mar 22 2023