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 A119091 #8 Mar 26 2023 23:26:37
%S A119091 6,66,666,990,66066,69006,600060,609960,690900,969606666,60690060606,
%T A119091 96960006066,66999966009690066,96606660096609006,6900969009006996666,
%U A119091 9906090099969060666906,6666999999969090090996690,60969009000966690090069999000606,660690009096060660069069660609900960960
%N A119091 Triangular numbers composed of digits {0,6,9}.
%H A119091 Giovanni Resta, <a href="http://www.numbersaplenty.com/tr/tr069.html">Tridigital Triangular Numbers</a>
%F A119091 a(n) = A000217(A119092(n)). - _Tyler Busby_, Mar 26 2023
%Y A119091 Cf. A000217, A058452, A119092. See A119033 for a table of cross-references.
%K A119091 nonn,base
%O A119091 1,1
%A A119091 _Giovanni Resta_, May 10 2006
%E A119091 a(18)-a(19) from _Tyler Busby_, Mar 26 2023